build: Omit -I... and -L... for standard paths in pkgconfig file

* configure.ac (GPGME_CONFIG_CFLAGS): Prefix with -I${includedir} for
non-standard include paths.
(GPGME_CONFIG_LIBS): Prefix with -L${libdir} for non-standard library
paths.
* src/gpgme.pc.in (Cflags): Remove hard-coded -I${includedir}.
(Libs): Remove hard-coded -L${libdir}.
--

This helps prevent problems when building/linking something that depends
on gpgme (installed in standard path) and some other library (installed
in a standard path and a custom path).

See T6136 for related changes in libgpg-error.

configure.ac

@@ -850,11 +850,45 @@ AH_BOTTOM([
 ])
 
 
-# Substitution used for gpgme-config
+# Substitution used for gpgme-config and gpgme.pc
 GPGME_CONFIG_LIBS="-lgpgme"
 GPGME_CONFIG_CFLAGS=""
 GPGME_CONFIG_HOST="$host"
 GPGME_CONFIG_AVAIL_LANG="$enabled_languages"
+
+case "$includedir" in
+  /usr/include|/include) ;;
+  '${prefix}/include')
+    if test "$prefix" != / -a "$prefix" != /usr; then
+      if test -z "$GPGME_CONFIG_CFLAGS"; then
+        GPGME_CONFIG_CFLAGS="-I\${includedir}"
+      else
+        GPGME_CONFIG_CFLAGS="-I\${includedir} $GPGME_CONFIG_CFLAGS"
+      fi
+    fi
+    ;;
+  *)
+    if test -z "$GPGME_CONFIG_CFLAGS"; then
+      GPGME_CONFIG_CFLAGS="-I\${includedir}"
+    else
+      GPGME_CONFIG_CFLAGS="-I\${includedir} $GPGME_CONFIG_CFLAGS"
+    fi
+    ;;
+esac
+case "$libdir" in
+  /usr/lib|/usr/lib64|/lib|/lib64) ;;
+  '${exec_prefix}/lib'|'${exec_prefix}/lib64')
+    if test "$exec_prefix" = "NONE"; then
+      if test "$prefix" != / -a "$prefix" != /usr; then
+        GPGME_CONFIG_LIBS="-L\${libdir} $GPGME_CONFIG_LIBS"
+      fi
+    elif test "$exec_prefix" != / -a "$exec_prefix" != /usr; then
+      GPGME_CONFIG_LIBS="-L\${libdir} $GPGME_CONFIG_LIBS"
+    fi
+    ;;
+  *) GPGME_CONFIG_LIBS="-L\${libdir} $GPGME_CONFIG_LIBS" ;;
+esac
+
 AC_SUBST(GPGME_CONFIG_API_VERSION)
 AC_SUBST(GPGME_CONFIG_LIBS)
 AC_SUBST(GPGME_CONFIG_CFLAGS)

src/gpgme.pc.in

@@ -9,6 +9,6 @@ Name: gpgme
 Description: GnuPG Made Easy to access GnuPG
 Requires.private: gpg-error, libassuan
 Version: @PACKAGE_VERSION@
-Cflags: -I${includedir} @GPGME_CONFIG_CFLAGS@
-Libs: -L${libdir} @GPGME_CONFIG_LIBS@
+Cflags: @GPGME_CONFIG_CFLAGS@
+Libs: @GPGME_CONFIG_LIBS@
 URL: https://www.gnupg.org/software/gpgme/index.html