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>