ssp: appease -Wgnu-statement-expression-from-macro-expansion

It's a stupid warning, but some ports enable it by default and were
already defining _FORTIFY_SOURCE, thus exposing the new macros
immediately. This at least fixes the libfido2 build, perhaps others as
well.

While we're here, fix a fresh build of stand w/ FORTIFY_SOURCE enabled
by not pulling in the ssp headers if _STANDALONE is defined. We do not
have runtime support in libsa as of the time of writing.

Reported by: netchild
Sponsored by: Stormshield
Sponsored by: Klara, Inc.