CMake: Correctly detect -Wno-... support for gcc

gcc does not emit a warning if an unknown -Wno-... flag is specified on the
command line, thus the ENABLE_IF_SUPPORTED macro unconditionally enabled
-Wno-... flags for gcc. Unfortunately, gcc _does_ emit a warning for the
unrecognized compiler option if another warning is emitted in the same
compilation unit. This is now fixed by always querying for the non-negated
version, i.e. -Wfoo instead of -Wno-foo.
Merge pull request #297 from tamiko/fix_wno-xxx_detection_for_gcc

CMake: Correctly detect -Wno-... support for gcc