build: Make gpgme.m4 use gpgrt-config with *.pc.

* src/gpgme.m4 (_AM_PATH_GPGME_CONFIG): Use gpgrt-config with gpgme.pc
when possible.
(AM_PATH_GPGME_GLIB): Likewise with gpgme-glib.pc.

Keeping AM_PATH_GPGME_PTHREAD, as is, untouched.

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