c-stack: fix regression on Irix 5.3 from 2008-06-21

c-stack: fix regression on Irix 5.3 from 2008-06-21

* m4/c-stack.m4 (gl_PREREQ_C_STACK): Move check for
sa_sigaction...
* m4/sigaction.m4 (gl_PREREQ_SIG_HANDLER_H): ...here.
(gl_PREREQ_SIGACTION): Depend on sig-handler.h prereq's.
* m4/signal_h.m4 (gl_SIGNAL_H_DEFAULTS): Set the default.
* modules/signal (Makefile.am): Use the value.
* lib/signal.in.h (sa_sigaction) [HAVE_SIGACTION
&& !HAVE_STRUCT_SIGACTION_SA_SIGACTION]: Define.
* doc/posix-headers/signal.texi (signal.h): Document this
portability issue.
* doc/posix-functions/sigaction.texi (sigaction): Likewise.
Reported by Tom G. Christensen.

Signed-off-by: Eric Blake <ebb9@byu.net>