AC_CHECK_ALIGNOF: fix cross-compilation bug with newer gcc

AC_CHECK_ALIGNOF: fix cross-compilation bug with newer gcc

* doc/autoconf.texi (Default Includes, Particular Functions)
(Header Portability):
* lib/autoconf/c.m4 (AC_LANG_FUNC_LINK_TRY(C)):
* lib/autoconf/headers.m4 (AC_HEADER_STDC):
* lib/autoconf/types.m4 (_AC_CHECK_ALIGNOF):
* lib/m4sugar/m4sugar.m4 (m4_require) [comment only]:
Assume the existence of the C89 freestanding headers <float.h>,
<limits.h>, <stdarg.h>, <stddef.h>, as that's safe nowadays.
This is less likely to run into gotchas, and should fix a
cross-compilation bug with newer GCC reported by Myke Frysinger in
<http://lists.gnu.org/archive/html/bug-autoconf/2012-09/msg00001.html>.