Make NAG+OpenMP work

Sebastian Müllerrequested to merge
nag_omp_fixes into main

NAG had some special requirements to work with OpenMP:

  • atomic capture needs an inplace operation (see mo_dag)
  • linking requires setting flags explicitly
  • had to increase minimum CMake version to 3.18 to use LINK_LANGUAGE generator expression
    • all EVE tool chains used by us have this

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