c-stack: test that stack overflow can be caught

c-stack: test that stack overflow can be caught

* m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Also check
that platform allows handling stack overflow; at least OS/2 EMX
has sigaltstack, but crashes before transferring control to
handler on stack overflow.
* lib/c-stack.c [HAVE_SIGALTSTACK]: Adjust conditions to also
check for HAVE_STACK_OVERFLOW_HANDLING.
Reported by Elbert Pol.

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