c-stack: avoid compiler optimizations when provoking overflow * m4/c-stack.m4 (AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Make recursion harder to optimize, to ensure a stack overflow occurs. * tests/test-c-stack.c (recurse): Likewise. Borrowed from libsigsegv. Signed-off-by: Eric Blake <ebb9@byu.net>