m4: Support cross-compiling better.
* src/gpg-error.m4 (GPGRT_CONFIG): Prefer the one under $prefix. (sed expression): Better compatibility to POSIX sed.
- GnuPG-bug-id: T5365
- Reported-by: Jay Freeman (saurik)
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>