message: fix compatibility with older verions of TBB
diff: @@ -56,7 +56,11 @@ namespace ospcommon {
  int num_threads()
  {
  #if defined(OSPRAY_TASKING_TBB)
  +# if TBB_INTERFACE_VERSION >= 9100
  return tbb::this_task_arena::max_concurrency();
  +# else
  + return tbb::task_scheduler_init::default_num_threads();
  +# endif
  #elif defined(OSPRAY_TASKING_CILK)
  return numThreads >= 0 ? std::thread::hardware_concurrency();
  #elif defined(OSPRAY_TASKING_OMP)
  