[sudo-workers] Improve stack smashing protector check

[Todd C. Miller]

I see the problem now, it's fixed by:
    http://www.sudo.ws/repos/sudo/rev/ab4f94aac7de

With that change and gcc with a specs file that has an empty link_ssp
section I see:

checking for compiler stack protector support... no

Whereas before I got:

checking for compiler stack protector support... -fstack-protector-all

 - todd