sixtracklib: update and streamline top-level sixtracklib CMakeLists.txt file after the refactoring
sixtracklib: update and streamline top-level sixtracklib CMakeLists.txt file after the refactoring

SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib