(ENOTSUP): Define if errno.h doesn't.
(SA_NODEFER, SA_ONSTACK, SA_RESETHAND, SA_SIGINFO, SIGSTKSZ, _SC_PAGESIZE,
ALTERNATE_STACK_SIZE, stack_t, sigaltstack): Remove; we now assume them
all when HAVE_XSI_STACK_OVERFLOW_HEURISTIC, so we don't need substitutes.
(<ucontext.h>): Include only if HAVE_XSI_STACK_OVERFLOW_HEURISTIC.
(alternate_signal_stack): Now of size SIGSTKSZ, not ALTERNATE_STACK_SIZE.
(segv_handler): Simplify, under the assumption that
HAVE_XSI_STACK_OVERFLOW_HEURISTIC is nonzero.
(c_stack_action): Likewise.
(exit_failure) [DEBUG]: Initialize to 0, not 1.
(recurse, main) [DEBUG]: Remove main args.
