Merge branch 'issue/fix_HAVE_MPI_Wundef_warning' into 'master'

Simon Praetoriusrequested to merge
cherry-pick-688413e1 into releases/2.8

Fix HAVE_MPI undefined warnings (with -Wundef flag)

See merge request !992 (merged)

(cherry picked from commit 688413e1)

db5a2d3a Fix HAVE_MPI undefined warnings (with -Wundef flag)

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