gpg-error-config: Fix PKG_CONFIG_LIBDIR support.

* configure.ac (pkg_config_libdir): New.
* src/gpg-error-config-new.in: Use pkg_config_libdir.

PKG_CONFIG_LIBDIR is not mere libdir but should include
'pkgconfig'.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>