gpg-error.m4: Factor out _AM_PATH_GPGRT_CONFIG.
* src/gpg-error.m4 (_AM_PATH_POSSIBLE_GPG_ERROR_CONFIG): New. (_AM_PATH_GPGRT_CONFIG): New. (AM_PATH_GPG_ERROR): Use _AM_PATH_POSSIBLE_GPG_ERROR_CONFIG and _AM_PATH_GPGRT_CONFIG.
- GnuPG-bug-id: T6273
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>