Flang support

On Atos HPC, we have Flang compiler availability. I tried to get it working ad hoc, but it did not succeed.

I added this to the top-level CMakeList.txt (works):

if(CMAKE_Fortran_COMPILER_ID MATCHES "Flang")
  target_compile_definitions(${LIB_NAME} PRIVATE "FLANG")
  target_compile_options(${LIB_NAME} PRIVATE
    -Mfreeform # -static-flang-libs 
    $<$<CONFIG:DEBUG>:-g0 -O0 -gline-directives-only>
    $<$<CONFIG:RELEASE>:>-Ofast
  )
endif()

Current issues are: isatty and gethostname cannot be used in mo_logging.F90, I could not figure out how to import these in FLang. I think it is a rather low hanging fruit to support Flang, though.