bring back the libgnutls-config use for checking for GnuTLS if pkg-config

doesn't work, to better fine older gnutls installs

configure.ac

@@ -1548,6 +1548,7 @@ if test "$OPENSSL_ENABLED" != "1"; then
 
     addld=""
     if test "x$OPT_GNUTLS" = "xyes"; then
+      dnl this is with no partiular path given
       CURL_CHECK_PKGCONFIG(gnutls)
 
       if test "$PKGCONFIG" != "no" ; then
@@ -1556,15 +1557,35 @@ if test "$OPENSSL_ENABLED" != "1"; then
         addcflags=`$PKGCONFIG --cflags-only-I gnutls`
         version=`$PKGCONFIG --modversion gnutls`
         gtlslib=`echo $addld | $SED -e 's/-L//'`
+      else
+        dnl without pkg-config, we try libgnutls-config as that was how it
+        dnl used to be done
+        check=`libgnutls-config --version 2>/dev/null`
+        if test -n "$check"; then
+          addlib=`libgnutls-config --libs`
+          addcflags=`libgnutls-config --cflags`
+          version=`libgnutls-config --version`
+          gtlslib=`libgnutls-config --prefix`/lib$libsuff
+        fi
       fi
     else
-      dnl without pkg-config, we guess a lot!
+      dnl this is with a given path, first check if there's a libgnutls-config
+      dnl there and if not, make an educated guess
+      check=`$OPT_GNUTLS/libgnutls-config --version 2>/dev/null`
+      if test -n "$check"; then
+        addlib=`$OPT_GNUTLS/libgnutls-config --libs`
+        addcflags=`$OPT_GNUTLS/libgnutls-config --cflags`
+        version=`$OPT_GNUTLS/libgnutls-config --version`
+        gtlslib=`$OPT_GNUTLS/libgnutls-config --prefix`/lib$libsuff
+      else
+        dnl without pkg-config and libgnutls-config, we guess a lot!
         addlib=-lgnutls
         addld=-L$OPT_GNUTLS/lib$libsuff
         addcflags=-I$OPT_GNUTLS/include
         version="" # we just don't know
         gtlslib=$OPT_GNUTLS/lib$libsuff
       fi
+    fi
 
     if test -z "$version"; then
       dnl lots of efforts, still no go