[!736] cmake: separate commandline args to list

Merge branch 'fix/mpi-cmake' into 'master'

ref:core/dune-common 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]

See merge request [!736]

  [!709]: gitlab.dune-project.org/NoneNone/merge_requests/709
  [!736]: gitlab.dune-project.org/core/dune-common/merge_requests/736

cmake/modules/DuneMPI.cmake

@@ -58,7 +58,8 @@ function(add_dune_mpi_flags)
     set(_prefix TARGET)
   endif()
   if(MPI_C_FOUND)
-    set_property(${_prefix} ${ADD_MPI_UNPARSED_ARGUMENTS} APPEND PROPERTY COMPILE_OPTIONS ${MPI_DUNE_COMPILE_FLAGS})
+    separate_arguments(MPI_DUNE_COMPILE_FLAGS_LIST UNIX_COMMAND ${MPI_DUNE_COMPILE_FLAGS})
+    set_property(${_prefix} ${ADD_MPI_UNPARSED_ARGUMENTS} APPEND PROPERTY COMPILE_OPTIONS ${MPI_DUNE_COMPILE_FLAGS_LIST})
     set_property(${_prefix} ${ADD_MPI_UNPARSED_ARGUMENTS} APPEND PROPERTY COMPILE_DEFINITIONS ENABLE_MPI=1
       MPICH_SKIP_MPICXX MPIPP_H)
     if(NOT (ADD_MPI_SOURCE_ONLY OR ADD_MPI_OBJECT))