build: Supply CC to invoke gpg-error-config.

* src/gpg-error.m4: Add CC when invoking gpg-error-config.
* src/gpgrt.m4: Likewise.

For standard cross build, the use of --with-libgpg-error-prefix will
not be needed, as the script automatically finds the place.

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