Fix altstack check to work again on OSX with -O2.

Fix altstack check to work again on OSX with -O2.

	* configure.in: The altstack check depends on raising
	a sigsegv to verify if it works or not. A sigsegv, by its
	own nature is born out of undefined behavior. Such a thing
	the almighty C compilers are free to optimize in whatever
	pleases them the most.

	GCC, under -O2, when it can prove a given memory store will
	lead to undefined behavior will simply remove it leaving no
	trace back. The solution was to use a variable gcc can't get
	frisky with.

	This raises altstack on OSX back from the dead.