build: Prefer gpgrt-config when available.
* src/libgcrypt.m4: Overriding the decision by --with-libgcrypt-prefix, use gpgrt-config libgcrypt when gpgrt-config is available.
Backport the master commit of:
0dcb7e05c9e1c9c2a23abe0a0390680741b61414
This may offer better migration.
- GnuPG-bug-id: T5034
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>