Silence a gcc warning "array subscript ... is partly outside array bounds".
* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add flexmember.
* gettext-tools/src/xg-arglist-parser.h (struct arglist_parser): Use
FLEXIBLE_ARRAY_MEMBER.
* gettext-tools/src/xg-arglist-parser.c: Include flexmember.h.
(arglist_parser_alloc, arglist_parser_clone): Use FLEXNSIZEOF instead of
hand-made size computations.
