- easier to ensure that flags are consistent
- required if there is no system openmp and libomp or libgomp are only found in the clang hierarchy STYLE: add some notes in the openmp rules. - the _OPENMP macro is now used in low-level testing files
- allows enabling new warnings without causing messy output with older clang versions
- initial settings for ADIOS2, linuxARM64Gcc, linuxARM64Clang