adjust to new cmake macros
    
    The diff is viewed best against 663cf5c ("CV_TRACE OFF", 2020-07-27)
    which was the last commit before various failing attempts to adjust to
    the change of %cmake.
    
    It shows that this is a minimal change which restores the previous build
    dir. One could adjust to the new default build dir instead, but:
    - this would require more changes to path variables
    - we are not running multiple build from the source unmodified source
      (we even patch the source)