gpgrt-config: Prepend PKG_CONFIG_LIBDIR to PKG_CONFIG_PATH.
* src/gpgrt-config: Handle empty PKG_CONFIG_LIBDIR config, which should ignore --libdir option. Otherwise, prepend PKG_CONFIG_LIBDIR (instead of appending) to PKG_CONFIG_PATH.
- Suggested-by: Andre Heinecke <aheinecke@intevation.de>
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>