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>