build: Prefer gpgrt-config when available.

* src/gpgme.m4: Overriding the decision by --with-gpgme-prefix,
use gpgrt-config gpgme when gpgrt-config is available.

This may offer better migration.

• GnuPG-bug-id: T5034
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>