GPGME support: configure should no longer rely on gpgme-config, which has been dropped

With recent GPGME on Debian, Mutt no longer enables GPGME automatically, because gpgme-config has been removed due to a lack of support by libgpg-error. Actually this is because gpg-error-config was removed from GnuPG installations by default. These are recent changes upstream, so I would expect that more than Debian would be affected soon. So Mutt's configure should no longer rely on gpgme-config.