Disable -Werror for warn_unused_result.

There is no way to disable warn_unused_result and glibc (or the gcc
fortification patches) add that to write in higher optimisation
levels.  We'll take the warning, but not make it fatal.

See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509 for the relevant
gcc bug report.
Only use -Wno-error=unused-result if the compiler supports it

Yay, gcc.  Or something.
Typos-r-us
Only add -Wno-unused-result if supported.