(AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Limit stack size

(AC_SYS_XSI_STACK_OVERFLOW_HEURISTIC): Limit stack size
to 1MB, so as not to render systems with no stack size limit (e.g.,
linux-2.2.x) unusable.  Suggestion and code from Bruno Haible.
Include <unistd.h>.  On some systems,
it is required for the definition of _SC_PAGESIZE.