cmake broken when TBB is not found.

When TBB is disabled by

-DCMAKE_DISABLE_FIND_PACKAGE_TBB=TRUE

then the cmake process in dune-common fails with

CMake Error at dune/common/CMakeLists.txt:31 (add_dune_tbb_flags): Unknown CMake command "add_dune_tbb_flags".

Guarding the function call with

if(TBB_FOUND)
  add_dune_tbb_flags(dunecommon)
endif()

fixes the problem. However, it may be desirable to have this if inside the function 'add_dune_tbb_flags'. It did not seem straight forward to fix this.