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

DuneMPI: append flags to `COMPILE_OPTIONS` instead of `COMPILE_FLAGS`

See merge request core/dune-common!499

cmake/modules/DuneMPI.cmake

@@ -58,7 +58,7 @@ function(add_dune_mpi_flags)
     set(_prefix TARGET)
   endif()
   if(MPI_C_FOUND)
-    set_property(${_prefix} ${ADD_MPI_UNPARSED_ARGUMENTS} APPEND PROPERTY COMPILE_FLAGS ${MPI_DUNE_COMPILE_FLAGS})
+    set_property(${_prefix} ${ADD_MPI_UNPARSED_ARGUMENTS} APPEND PROPERTY COMPILE_OPTIONS ${MPI_DUNE_COMPILE_FLAGS})
     set_property(${_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))