cmake: separate commandline args to list

Providing space-separated MPI_DUNE_COMPILE_FLAGS would previously add them as a single list entry to the cmake list COMPILE_OPTIONS and thus ruin the command-line having something like

/usr/bin/c++ "-arg1 -arg2" -other -args

Supersedes !709 (closed)

changed milestone to %DUNE 2.7.0

merged

mentioned in commit 13411d62