Enable the format_arg attribute also on clang on Mac OS X.
Reported by Tom Tromey <tom@tromey.com>
in <https://lists.gnu.org/archive/html/bug-gettext/2018-07/msg00000.html>.
* gettext-runtime/intl/libgnuintl.in.h (_INTL_MAY_RETURN_STRING_ARG): On macOS,
use attribute __format_arg__ if the compiler is based on clang >= 3.0.
