Merge branch 'FindMPI-restore-flags-string' into release-3.12

Merge-request: !2368

Modules/FindMPI.cmake

@@ -1680,7 +1680,7 @@ foreach(LANG IN ITEMS C CXX Fortran)
     set(MPI_${LANG}_INCLUDE_PATH "${MPI_${LANG}_INCLUDE_DIRS}")
     unset(MPI_${LANG}_COMPILE_FLAGS)
     if(MPI_${LANG}_COMPILE_OPTIONS)
-      set(MPI_${LANG}_COMPILE_FLAGS "${MPI_${LANG}_COMPILE_OPTIONS}")
+      list(JOIN MPI_${LANG}_COMPILE_FLAGS " " MPI_${LANG}_COMPILE_OPTIONS)
     endif()
     if(MPI_${LANG}_COMPILE_DEFINITIONS)
       foreach(_MPI_DEF IN LISTS MPI_${LANG}_COMPILE_DEFINITIONS)