Fix moc / rcc execution on macOS

Currently when moc is executed by CMake, it does not generate a moc_predefs.h file, which is important for determining the current OS define like Q_OS_MACOS, as well as other defines. This in turn leads to compilation errors because moc generates code that it shouldn't (in the platformsupport plugin for example).

Relevant work done is here, but there are still some issues:
https://codereview.qt-project.org/#/c/253155/