stddef: Avoid conflict with system-defined max_align_t. The configure-determined HAVE_MAX_ALIGN_T may not always be accurate. Reported by Werner Lemberg <wl@gnu.org> in <https://lists.gnu.org/archive/html/bug-gnulib/2017-08/msg00185.html>. * lib/stddef.in.h (rpl_max_align_t): Renamed from max_align_t. (max_align_t): Define as a macro. (GNULIB_defined_max_align_t): New macro. Guards against multiple definitions of rpl_max_align_t in different copies of gnulib-generated <stddef.h>.