Introduce OPENMP_ENABLE_LIBOMPTARGET which defaults to OFF at the moment.
libomptarget is not yet ready for prime time:
- Offloading to NVIDIA GPUs is not completed yet (compiler, device RTL)
- The generic ELF plugin for offloading to the host (meant for testing) uses a single instance of the OpenMP runtime (libomp). That is why omp_is_initial_device() returns 1 which makes the tests fail.
Because of these reasons, we want to disable building (and testing!) for release 5.0.