Merge from gnulib (a couple more changes)
Replace union hack with C11-style max_align_t

* admin/merge-gnulib (GNULIB_MODULES): Add stddef, for max_align_t.
* lib/gnulib.mk: Regenerate.
* src/alloc.c (XMALLOC_BASE_ALIGNMENT): Use max_align_t instead of
a hack involving a union.