Weaken -fstack-protector configure test to a compile test

[Roland McGrath]

I think this is probably fine.  

But one thing that comes to mind is if the old check might also have been
verifying that GCC was not generating some external library call that is
missing.

Also, the old test actually compiled a function whereas now it's an empty
input file.  So if there is some error/warning that only arises when the
compiler actually has anything to do, we could be missing it.

Those both seem unlikely to have been real issues, but it's worth
raising the questions.


Thanks,
Roland