stdnoreturn: port to newer GCCs

stdnoreturn: port to newer GCCs

* m4/stdnoreturn.m4 (gl_STDNORETURN_H): Avoid problems with
bleeding-edge GCC that complains about 'int _Noreturn foo (void);'.
Problem reported by Jim Meyering in
<http://lists.gnu.org/archive/html/bug-gnulib/2012-08/msg00121.html>.
Also, rename the 'test' function to a void a clash with the
already-supplied 'main' function; this fixes a bug that incorrectly
rejected GCC 4.7.1's <stdnoreturn.h>.
* doc/posix-headers/stdnoreturn.texi (stdnoreturn.h):
Document GCC problem.