libcurl.pc: Merge Libs.private into Libs for static-only builds

A project being built entirely statically will call pkg-config with
--static, which utilises the Libs.private field. Conversely it will
not use --static when not being built entirely statically, even if
there is only a static build of libcurl available. This will most
likely cause the build to fail due to underlinking unless we merge the
Libs fields.

Consider that this is what the Meson build system does when it
generates pkg-config files.

I have also reflected this in the --libs argument of curl-config even
though REQUIRE_LIB_DEPS always seems to be "yes" anyway.

Closes #5373

CMakeLists.txt

@@ -1391,13 +1391,6 @@ set(CONFIGURE_OPTIONS       "")
 # TODO when to set "-DCURL_STATICLIB" for CPPFLAG_CURL_STATICLIB?
 set(CPPFLAG_CURL_STATICLIB  "")
 set(CURLVERSION             "${CURL_VERSION}")
-if(BUILD_SHARED_LIBS)
-  set(ENABLE_SHARED         "yes")
-  set(ENABLE_STATIC         "no")
-else()
-  set(ENABLE_SHARED         "no")
-  set(ENABLE_STATIC         "yes")
-endif()
 set(exec_prefix             "\${prefix}")
 set(includedir              "\${prefix}/include")
 set(LDFLAGS                 "${CMAKE_SHARED_LINKER_FLAGS}")
@@ -1410,6 +1403,15 @@ foreach(_lib ${CMAKE_C_IMPLICIT_LINK_LIBRARIES} ${CURL_LIBS})
     set(LIBCURL_LIBS          "${LIBCURL_LIBS} -l${_lib}")
   endif()
 endforeach()
+if(BUILD_SHARED_LIBS)
+  set(ENABLE_SHARED         "yes")
+  set(ENABLE_STATIC         "no")
+  set(LIBCURL_NO_SHARED     "")
+else()
+  set(ENABLE_SHARED         "no")
+  set(ENABLE_STATIC         "yes")
+  set(LIBCURL_NO_SHARED     "${LIBCURL_LIBS}")
+endif()
 # "a" (Linux) or "lib" (Windows)
 string(REPLACE "." "" libext "${CMAKE_STATIC_LIBRARY_SUFFIX}")
 set(prefix                  "${CMAKE_INSTALL_PREFIX}")

configure.ac

@@ -4727,6 +4727,13 @@ dnl to let curl-config output the static libraries correctly
 ENABLE_STATIC="$enable_static"
 AC_SUBST(ENABLE_STATIC)
 
+dnl merge the pkg-config Libs.private field into Libs when static-only
+if test "x$enable_shared" = "xno"; then
+  LIBCURL_NO_SHARED=$LIBCURL_LIBS
+else
+  LIBCURL_NO_SHARED=
+fi
+AC_SUBST(LIBCURL_NO_SHARED)
 
 dnl
 dnl For keeping supported features and protocols also in pkg-config file

curl-config.in

@@ -160,7 +160,7 @@ while test $# -gt 0; do
         else
            CURLLIBDIR=""
         fi
-        if test "X@REQUIRE_LIB_DEPS@" = "Xyes"; then
+        if test "X@ENABLE_SHARED@" = "Xno" -o "X@REQUIRE_LIB_DEPS@" = "Xyes"; then
           echo ${CURLLIBDIR}-lcurl @LIBCURL_LIBS@
         else
           echo ${CURLLIBDIR}-lcurl

libcurl.pc.in

@@ -34,6 +34,6 @@ Name: libcurl
 URL: https://curl.haxx.se/
 Description: Library to transfer files with ftp, http, etc.
 Version: @CURLVERSION@
-Libs: -L${libdir} -lcurl
+Libs: -L${libdir} -lcurl @LIBCURL_NO_SHARED@
 Libs.private: @LIBCURL_LIBS@
 Cflags: -I${includedir} @CPPFLAG_CURL_STATICLIB@