intl: Improve declaration of 'struct binding'.
* autogen.sh (GNULIB_MODULES_LIBINTL): Add flexmember.
* gettext-runtime/intl/gettextP.h (struct binding): Use FLEXIBLE_ARRAY_MEMBER
instead of ZERO.
* gettext-runtime/intl/bindtextdom.c: Include flexmember.h.
(offsetof): Remove fallback definition.
(set_binding_values): Use FLEXNSIZEOF.
