m4: Fix behavior with older gpg-error-config and gpgrt-config.

* src/gpg-error.m4 (_AM_PATH_GPGRT_CONFIG): Set the variable
gpg_error_config_version on error with gpgrt-config.

When detecting older gpgrt-config (<= 1.35), make sure to set
the variable.

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