Fix logic bug and make language explicit

The operator precedence in CMake can be surprising, so add parenthesis
to make in unambigious. Also, make it more explicit from which language
we get the default options. (They are all set from the settings given
for C++; why would have different optimization options for C and C++?)
Clear default flags in an (im)proper way

The CMAKE_NOT_USING_CONFIG_FLAGS option is really only settable by a
user that doesn't want _DEBUG or _RELEASE flags to not have effect.
If we want other flags than the platform default, we must do hairy
things like clearing them from the cache (overriding the user's
seletion if it was the same as the platform default).