Disabling TBB check fails cmake

The fact that we are having add_dune_tbb_flags within the TBB find module breaks the CMake check disabling mechanism: If you run cmake with -DCMAKE_DISABLE_FIND_PACKAGE_TBB=1 the find module is never executed and any later uses of add_dune_tbb_flags result in an Unknown CMake command error. My proposed solution is to move the add function out of the find module.