build: Fix m4/gpg-error.m4.

* m4/gpg-error.m4: Unset GPGRT_CONFIG when it doesn't work well.

With older version of libgpg-error (< 1.33), the gpgrt-config
script doesn't support .pc files.

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