MSVC leak detection changes.

I checked, the only supported way of checking which mode MSVC is currently compiling in, is using `_DEBUG`. It allows the user to set other macros independently. The leak detection functionality is only enabled when `_DEBUG` is set, which means that using any other macro to detect this functionality would be misguided. Ultimately, it doesn't matter what macros QtCreator sets, as long as it doesn't set _MSC_VER, it will just ignore this entirely.