Problem reported by Eric Blake in:
https://lists.gnu.org/r/bug-gnulib/2021-04/msg00181.html
* lib/cdefs.h: Make this closer to glibc, so that we can
merge Gnulib into glibc here.
(__attribute_maybe_unused__): Make identical to glibc’s
definition, though with an additional comment.
(__attribute_nonnull__): Add clarifying comment.
Make inclusion of bits/wordsize.h, bits/long-double.h
dependent on __GLIBC__, not on !__WORDSIZE.
* lib/libc-config.h: Use __attribute_nonnull__,
not __attribute_maybe_unused__, to detect whether
to include sys/cdefs.h.