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>.