gpg-error-config: Support PKG_CONFIG_LIBDIR.

* src/gpg-error-config-new.in: Use PKG_CONFIG_LIBDIR if specified.

This change allows a distribution (like Debian) offering
<triplet>-gpg-error-config wrapper script which invokes
gpg-error-config with host specific PKG_CONFIG_LIBDIR.

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