`FindTBB.cmake` conflicts with other project's `FindTBB.cmake`

We just had a case that we wanted to build with PACXX, which needs to look for TBB, so it ships a FindTBB.cmake. If that version of FindTBB.cmake is selected over Dune's FindTBB.cmake while building Dune, certain cmake functions (add_dune_tbb_flags()) are missing.