Fix warning supression for Linux/OSX, this was broken after the first round of...

Fix warning supression for Linux/OSX, this was broken after the first round of cmake changes on Windows.
-Wno-unused-but-set-variable/-Wno-unused-variable moved from just Linux to also OSX (clang) as there's a lot of those
in the code where newer xcode complains. Fixing all those places is going beyond the scope of modernizing cmake.