DuneMPI: add spaces around linker flags

Same reasoning as 4063acee applies, however there is no LINK_OPTIONS that could be used instead of LINK_FLAGS 1. So just add some spaces to make sure that distinct options aren't joined by accident.

Only noticed after reconfiguring dune-common in an empty tree where it picked up a different set of options form a newer OpenMPI version...

enabled an automatic merge when the pipeline for 2c9a2c84 succeeds

merged

mentioned in commit 162f8dea

mentioned in commit 85bb4545

mentioned in merge request !504 (merged)