FindMPI: Restore MPI_<LANG>_COMPILE_FLAGS as a command-line string

Refactoring in commit v3.10.0-rc1~103^2~2 (FindMPI: Modernization from
ground up, 2017-04-25) accidentally left this variable set as a copy of
the `;`-list in `MPI_<LANG>_COMPILE_OPTIONS`, but the flags variable is
documented as a command-line string.  Restore it now.

Fixes: #18349

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)