build: Set GPGRT_CONFIG in gpg-error.m4.

* src/gpg-error.m4: Detect gpgrt-config to set GPGRT_CONFIG.

Set GPGRT_CONFIG when there is no gpg-error-config or gpg-error-config
reports version >= 1.33.

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