Cleanup import of TBB in CMake.

- Update FindSuiteSparse to use FindTBB.cmake to find TBB.
- Fix logic handling of TBB=ON if TBB is not found to continue with
  build after disabling TBB rather than throwing a fatal error.
- Use TBB_LIBRARIES instead of TBB_tbb_LIBRARY as a Ceres dependency,
  the former also includes the TBB malloc library.
- Add warning message about GPL licensing if TBB version < 2017.
- Add ‘TBB’ & ‘Mulithreading’ component options to find_package(Ceres),
  where ‘Mulithreading’ is equivalent to ‘TBB’ || ‘OpenMP’.

Change-Id: Ifc7f1d01b050ba6e2097ad1913b178805df4769a