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

`COMPILE_FLAGS` is a string, but `set_property(... APPEND ...)`
appends to a list.  If the list of compile flags is not empty this
results in a superfluous semicolon in the compile flags which will
break the compiler invocation.

For example, when `MPI_DUNE_COMPILE_FLAGS=-lpthread`, calling
`add_dune_mpi_flags` twice for the same target would result in the
compiler command containing `-lpthread;-lpthread`.

This patch makes use of the `COMPILE_OPTIONS` property instead which
is a list.

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))