Revert "use system cpp instead of ucpp"
    
    This reverts commit e6dc0f6267e83d4cf3c6960cd8d3f7fe8eb8bda7.
    
    Build fails with cpp 4.8.0. That is fixable by passing -P to cpp, but
    then idlc produces empty urd files (with module descriptions only). I
    have no idea why, because a randomly checked preprocessed file looks
    fine and idlc seems to parse it correctly. It even seems to dump the
    data...