gpg-error-config: Support "auto", architecture independent script.

* configure.ac (pkg_config_libdir): Set it "auto" when it looks
multiarch environment.
* src/gpg-error-config-new.in: Automatically detects the triplet.

The triplet detection mechanism is not sufficient. It's up to
distributions to apply more changes (in distribution specific way).

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