Build Script Fixes, master branch (2021.08.23.)

When updating the scripts to perform the build through cmake and not using make/ninja directly (!39481 (merged)), the passing of user-provided flags was not updated correctly in the code. Or at least it does not work now, with modern CMake versions anymore...

The "trick" was just to add -- to the CMake build commands, to clearly signal which flag(s) is/are going to the underlying build tool.