build: omit certain deps from libcurl.pc unless found via pkg-config

The idea of linking dependencies found to `libcurl.pc` turns out not
to work in practice in some cases.

Specifically: gss, ldap, mbedtls, libmsh3, rustls

A `.pc` may not work or be missing for a couple of reasons:
- not all build methods generate it: mbedTLS, Rustls
- generated file is broken: msh3
  Ref: https://github.com/nibanks/msh3/pull/225
- installed package flavour isn't shipping with one:
  FreeBSD GSS, OmniOS LDAP, macOS LDAP

The effect of such issues shall be subtle in theory, because
`libcurl.pc` normally lists these dependencies in the `Requires.private`
section meant for static linking. But, e.g. `pkg-config --exists`
requires these to be present, and builds sometimes use this check
regardless of build type. This bug is not present in `pkgconf`; it only
checks for them when `--static` is also passed.

Fix these by adding affected `.pc` references to `libcurl.pc` only when
we detected the dependency via `pkg-config`.

There are a few side-effects of this solution:
- references are never added for dependencies where curl doesn't
  implement `pkg-config` detection. These are:
  - autotools: ldap, mbedtls, msh3
  - cmake: ldap (pending #15273)
- generated `libcurl.pc` depends on the build-time environment.
- generated `libcurl.pc` depends on curl build tool (cmake, autotools).
- generated `libcurl.pc` depends on curl build implementation details.

Make an exception for GNU GSS, where I blindly guess that `gss.pc` is
always available, as no issues were reported.

Other, not mentioned, dependencies continue to be added regardless
of the detection method.

Reported-by: Harmen Stoppels, Thomas, Daniel Engberg, Andy Fiddaman
Fixes #15469
Fixes #15507
Fixes #15535
Fixes https://github.com/curl/curl/pull/15163#issuecomment-2473358444
Closes #15573

CMake/FindGSS.cmake

@@ -34,6 +34,7 @@
 # - `GSS_INCLUDE_DIRS`:  The GSS include directories.
 # - `GSS_LIBRARIES`:     The GSS library names.
 # - `GSS_LIBRARY_DIRS`:  The GSS library directories.
+# - `GSS_PC_REQUIRES`:   The GSS pkg-config packages.
 # - `GSS_CFLAGS`:        Required compiler flags.
 # - `GSS_VERSION`:       This is set to version advertised by pkg-config or read from manifest.
 #                        In case the library is found but no version info available it is set to "unknown"
@@ -212,6 +213,7 @@ if(NOT _GSS_FOUND)  # Not found by pkg-config. Let us take more traditional appr
 
         if(_GSS_INCLUDE_DIRS)
           set(GSS_FLAVOUR "GNU")
+          set(GSS_PC_REQUIRES "gss")
         endif()
       endif()
     endif()
@@ -266,16 +268,19 @@ else()
   # _GSS_MODULE_NAME set since CMake 3.16
   if(_GSS_MODULE_NAME STREQUAL _gnu_modname OR _GSS_${_gnu_modname}_VERSION)
     set(GSS_FLAVOUR "GNU")
+    set(GSS_PC_REQUIRES "gss")
     if(NOT _GSS_VERSION)  # for old CMake versions?
       set(_GSS_VERSION ${_GSS_${_gnu_modname}_VERSION})
     endif()
   elseif(_GSS_MODULE_NAME STREQUAL _mit_modname OR _GSS_${_mit_modname}_VERSION)
     set(GSS_FLAVOUR "MIT")
+    set(GSS_PC_REQUIRES "mit-krb5-gssapi")
     if(NOT _GSS_VERSION)  # for old CMake versions?
       set(_GSS_VERSION ${_GSS_${_mit_modname}_VERSION})
     endif()
   else()
     set(GSS_FLAVOUR "Heimdal")
+    set(GSS_PC_REQUIRES "heimdal-gssapi")
     if(NOT _GSS_VERSION)  # for old CMake versions?
       set(_GSS_VERSION ${_GSS_${_heimdal_modname}_VERSION})
     endif()

CMake/FindMSH3.cmake

@@ -34,6 +34,7 @@
 # - `MSH3_INCLUDE_DIRS`:  The msh3 include directories.
 # - `MSH3_LIBRARIES`:     The msh3 library names.
 # - `MSH3_LIBRARY_DIRS`:  The msh3 library directories.
+# - `MSH3_PC_REQUIRES`:   The msh3 pkg-config packages.
 # - `MSH3_CFLAGS`:        Required compiler flags.
 # - `MSH3_VERSION`:       Version of msh3.
 
@@ -45,6 +46,7 @@ if(CURL_USE_PKGCONFIG AND
 endif()
 
 if(MSH3_FOUND)
+  set(MSH3_PC_REQUIRES "libmsh3")
   string(REPLACE ";" " " MSH3_CFLAGS "${MSH3_CFLAGS}")
   message(STATUS "Found MSH3 (via pkg-config): ${MSH3_INCLUDE_DIRS} (found version \"${MSH3_VERSION}\")")
 else()

CMake/FindMbedTLS.cmake

@@ -36,6 +36,7 @@
 # - `MBEDTLS_INCLUDE_DIRS`:  The mbedTLS include directories.
 # - `MBEDTLS_LIBRARIES`:     The mbedTLS library names.
 # - `MBEDTLS_LIBRARY_DIRS`:  The mbedTLS library directories.
+# - `MBEDTLS_PC_REQUIRES`:   The mbedTLS pkg-config packages.
 # - `MBEDTLS_CFLAGS`:        Required compiler flags.
 # - `MBEDTLS_VERSION`:       Version of mbedTLS.
 
@@ -59,6 +60,7 @@ endif()
 if(MBEDTLS_FOUND AND MBEDX509_FOUND AND MBEDCRYPTO_FOUND)
   list(APPEND MBEDTLS_LIBRARIES ${MBEDX509_LIBRARIES} ${MBEDCRYPTO_LIBRARIES})
   list(REMOVE_DUPLICATES MBEDTLS_LIBRARIES)
+  set(MBEDTLS_PC_REQUIRES "mbedtls")
   string(REPLACE ";" " " MBEDTLS_CFLAGS "${MBEDTLS_CFLAGS}")
   message(STATUS "Found MbedTLS (via pkg-config): ${MBEDTLS_INCLUDE_DIRS} (found version \"${MBEDTLS_VERSION}\")")
 else()

CMake/FindRustls.cmake

@@ -34,6 +34,7 @@
 # - `RUSTLS_INCLUDE_DIRS`:  The Rustls include directories.
 # - `RUSTLS_LIBRARIES`:     The Rustls library names.
 # - `RUSTLS_LIBRARY_DIRS`:  The Rustls library directories.
+# - `RUSTLS_PC_REQUIRES`:   The Rustls pkg-config packages.
 # - `RUSTLS_CFLAGS`:        Required compiler flags.
 # - `RUSTLS_VERSION`:       Version of Rustls.
 
@@ -45,6 +46,7 @@ if(CURL_USE_PKGCONFIG AND
 endif()
 
 if(RUSTLS_FOUND)
+  set(RUSTLS_PC_REQUIRES "rustls")
   string(REPLACE ";" " " RUSTLS_CFLAGS "${RUSTLS_CFLAGS}")
   message(STATUS "Found Rustls (via pkg-config): ${RUSTLS_INCLUDE_DIRS} (found version \"${RUSTLS_VERSION}\")")
 else()

CMakeLists.txt

@@ -636,7 +636,7 @@ if(CURL_USE_MBEDTLS)
   set(USE_MBEDTLS ON)
   list(APPEND CURL_LIBS ${MBEDTLS_LIBRARIES})
   list(APPEND CURL_LIBDIRS ${MBEDTLS_LIBRARY_DIRS})
-  list(APPEND LIBCURL_PC_REQUIRES_PRIVATE "mbedtls")
+  list(APPEND LIBCURL_PC_REQUIRES_PRIVATE ${MBEDTLS_PC_REQUIRES})
   include_directories(SYSTEM ${MBEDTLS_INCLUDE_DIRS})
   link_directories(${MBEDTLS_LIBRARY_DIRS})
   if(MBEDTLS_CFLAGS)
@@ -726,7 +726,7 @@ if(CURL_USE_RUSTLS)
   set(USE_RUSTLS ON)
   list(APPEND CURL_LIBS ${RUSTLS_LIBRARIES})
   list(APPEND CURL_LIBDIRS ${RUSTLS_LIBRARY_DIRS})
-  list(APPEND LIBCURL_PC_REQUIRES_PRIVATE "rustls")
+  list(APPEND LIBCURL_PC_REQUIRES_PRIVATE ${RUSTLS_PC_REQUIRES})
   include_directories(SYSTEM ${RUSTLS_INCLUDE_DIRS})
   link_directories(${RUSTLS_LIBRARY_DIRS})
   if(RUSTLS_CFLAGS)
@@ -952,7 +952,7 @@ if(USE_MSH3)
   find_package(MSH3 REQUIRED)
   list(APPEND CURL_LIBS ${MSH3_LIBRARIES})
   list(APPEND CURL_LIBDIRS ${MSH3_LIBRARY_DIRS})
-  list(APPEND LIBCURL_PC_REQUIRES_PRIVATE "libmsh3")
+  list(APPEND LIBCURL_PC_REQUIRES_PRIVATE ${MSH3_PC_REQUIRES})
   include_directories(SYSTEM ${MSH3_INCLUDE_DIRS})
   link_directories(${MSH3_LIBRARY_DIRS})
   if(MSH3_CFLAGS)
@@ -1041,7 +1041,8 @@ if(NOT CURL_DISABLE_LDAP)
       list(APPEND CMAKE_REQUIRED_DEFINITIONS "-DLDAP_DEPRECATED=1")
       list(APPEND CMAKE_REQUIRED_LIBRARIES ${LDAP_LIBRARY})
       set(CURL_LIBS "${LDAP_LIBRARY};${CURL_LIBS}")
-      set(LIBCURL_PC_REQUIRES_PRIVATE "ldap;${LIBCURL_PC_REQUIRES_PRIVATE}")
+      # FIXME: uncomment once pkg-config-based detection landed: https://github.com/curl/curl/pull/15273
+      # set(LIBCURL_PC_REQUIRES_PRIVATE "${LDAP_PC_REQUIRES};${LIBCURL_PC_REQUIRES_PRIVATE}")
       if(HAVE_LIBLBER)
         list(APPEND CMAKE_REQUIRED_LIBRARIES ${LDAP_LBER_LIBRARY})
         set(CURL_LIBS "${LDAP_LBER_LIBRARY};${CURL_LIBS}")
@@ -1248,13 +1249,7 @@ if(CURL_USE_GSSAPI)
 
     list(APPEND CURL_LIBS ${GSS_LIBRARIES})
     list(APPEND CURL_LIBDIRS ${GSS_LIBRARY_DIRS})
-    if(GSS_FLAVOUR STREQUAL "GNU")
+    list(APPEND LIBCURL_PC_REQUIRES_PRIVATE ${GSS_PC_REQUIRES})
-      list(APPEND LIBCURL_PC_REQUIRES_PRIVATE "gss")
-    elseif(GSS_FLAVOUR STREQUAL "MIT")
-      list(APPEND LIBCURL_PC_REQUIRES_PRIVATE "mit-krb5-gssapi")
-    else()  # Heimdal
-      list(APPEND LIBCURL_PC_REQUIRES_PRIVATE "heimdal-gssapi")
-    endif()
     include_directories(SYSTEM ${GSS_INCLUDE_DIRS})
     link_directories(${GSS_LIBRARY_DIRS})
     if(GSS_CFLAGS)

acinclude.m4

@@ -565,7 +565,9 @@ AC_DEFUN([CURL_CHECK_LIBS_LDAP], [
       else
         LIBS="$curl_cv_ldap_LIBS $curl_cv_save_LIBS"
       fi
-      LIBCURL_PC_REQUIRES_PRIVATE="ldap $LIBCURL_PC_REQUIRES_PRIVATE"
+      if false; then
+        LIBCURL_PC_REQUIRES_PRIVATE="ldap $LIBCURL_PC_REQUIRES_PRIVATE"
+      fi
       AC_MSG_RESULT([$curl_cv_ldap_LIBS])
       ;;
   esac

configure.ac

@@ -2015,12 +2015,14 @@ if test x"$want_gss" = xyes; then
   AC_DEFINE(HAVE_GSSAPI, 1, [if you have GSS-API libraries])
   HAVE_GSSAPI=1
   curl_gss_msg="enabled (MIT Kerberos/Heimdal)"
+  link_pkgconfig=''
 
   if test -n "$gnu_gss"; then
     curl_gss_msg="enabled (GNU GSS)"
     LDFLAGS="$LDFLAGS $GSSAPI_LIB_DIR"
     LDFLAGSPC="$LDFLAGSPC $GSSAPI_LIB_DIR"
     LIBS="-lgss $LIBS"
+    link_pkgconfig=1
   elif test -z "$GSSAPI_LIB_DIR"; then
     case $host in
       *-apple-*)
@@ -2040,11 +2042,13 @@ if test x"$want_gss" = xyes; then
         elif test "$PKGCONFIG" != "no"; then
           gss_libs=`$PKGCONFIG --libs mit-krb5-gssapi`
           LIBS="$gss_libs $LIBS"
+          link_pkgconfig=1
         elif test -f "$KRB5CONFIG"; then
           dnl krb5-config doesn't have --libs-only-L or similar, put everything
           dnl into LIBS
           gss_libs=`$KRB5CONFIG --libs gssapi`
           LIBS="$gss_libs $LIBS"
+          link_pkgconfig=1
         else
           case $host in
             *-hp-hpux*)
@@ -2077,12 +2081,14 @@ if test x"$want_gss" = xyes; then
         ;;
     esac
   fi
-  if test -n "$gnu_gss"; then
+  if test -n "$link_pkgconfig"; then
-    LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE gss"
+    if test -n "$gnu_gss"; then
-  elif test "x$not_mit" = "x1"; then
+      LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE gss"
-    LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE heimdal-gssapi"
+    elif test "x$not_mit" = "x1"; then
-  else
+      LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE heimdal-gssapi"
-    LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE mit-krb5-gssapi"
+    else
+      LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE mit-krb5-gssapi"
+    fi
   fi
 else
   CPPFLAGS="$save_CPPFLAGS"
@@ -3761,7 +3767,9 @@ if test X"$want_msh3" != Xno; then
         CURL_LIBRARY_PATH="$CURL_LIBRARY_PATH:$DIR_MSH3"
         export CURL_LIBRARY_PATH
         AC_MSG_NOTICE([Added $DIR_MSH3 to CURL_LIBRARY_PATH])
-        LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE libmsh3"
+        if false; then
+          LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE libmsh3"
+        fi
         experimental="$experimental HTTP3"
       )
     ],

m4/curl-mbedtls.m4

@@ -104,7 +104,9 @@ if test "x$OPT_MBEDTLS" != xno; then
           AC_MSG_NOTICE([Added $mbedtlslib to CURL_LIBRARY_PATH])
         fi
       fi
-      LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE mbedtls"
+      if false; then
+        LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE mbedtls"
+      fi
     fi
 
   fi dnl mbedTLS not disabled

m4/curl-rustls.m4

@@ -112,6 +112,8 @@ if test "x$OPT_RUSTLS" != xno; then
       ;;
   esac
 
+  link_pkgconfig=''
+
   if test "$PKGTEST" = "yes"; then
 
     CURL_CHECK_PKGCONFIG(rustls, [$RUSTLS_PCDIR])
@@ -140,6 +142,7 @@ if test "x$OPT_RUSTLS" != xno; then
       dnl additional libs may be necessary.  Hope that we
       dnl don't need any.
       LIBS="$SSL_LIBS $LIBS"
+      link_pkgconfig=1
       ssl_msg="rustls"
       AC_DEFINE(USE_RUSTLS, 1, [if Rustls is enabled])
       AC_SUBST(USE_RUSTLS, [1])
@@ -176,7 +179,9 @@ if test "x$OPT_RUSTLS" != xno; then
         AC_MSG_NOTICE([Added $LIB_RUSTLS to CURL_LIBRARY_PATH])
       fi
     fi
-    LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE rustls"
+    if test -n "$link_pkgconfig"; then
+      LIBCURL_PC_REQUIRES_PRIVATE="$LIBCURL_PC_REQUIRES_PRIVATE rustls"
+    fi
   fi
 
   test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"