message: when using OpenMP, have ospcommon::tasking::async() spawn a std::thread
diff: #ifdef OSPRAY_TASKING_TBB
  # include <tbb/task.h>
  +#elif defined(OSPRAY_TASKING_OMP)
  +# include <thread>
  #elif defined(OSPRAY_TASKING_CILK)
  # include <cilk/cilk.h>
  #elif defined(OSPRAY_TASKING_INTERNAL)
  @@ -44,11 +46,14 @@ namespace ospcommon {
  auto *tbb_node =
  new(tbb::task::allocate_root())LocalTBBTask(std::forward<TASK_T>(fcn));
  tbb::task::enqueue(*tbb_node);
  +#elif defined(OSPRAY_TASKING_OMP)
  + std::thread thread(fcn);
  + thread.detach();
  #elif defined(OSPRAY_TASKING_CILK)
  cilk_spawn fcn();
  #elif defined(OSPRAY_TASKING_INTERNAL)
  detail::schedule_internal(std::forward<TASK_T>(fcn));
  -#else// OpenMP or Debug --> synchronous!
  +#else// Debug --> synchronous!
  fcn();
  #endif
  }
  