configure: Move -Wno-maybe-uninitialized check into gcc section

As another example of bizarre compiler behavior clang groks the
-Wmaybe-uninitialized option, but not -Wno-maybe-uninitialized
and spews a warning for every file that gets compiled.
avfilter/graphparser: Directly free filter memory if initialization fails
Remove #undefs for formerly forbidden system functions

The macros forbidding the system functions no longer exist, obviating
the need for the #undefs.