Merge branch 'bugfix/DuneMPI-use-COMPILE_OPTIONS' into 'master'

Ansgar Burchardtrequested to merge
cherry-pick-96ed065a into releases/2.6

DuneMPI: append flags to COMPILE_OPTIONS instead of COMPILE_FLAGS

See merge request !499 (merged)

(cherry picked from commit 96ed065a)

4063acee DuneMPI: append flags to COMPILE_OPTIONS instead of COMPILE_FLAGS

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