change MPIEXEC to MPIEXEC_EXECTUABLE; add commands for backwards compatibility

cmake/waLBerlaFunctions.cmake

@@ -368,7 +368,10 @@ function ( waLBerla_execute_test )
    endif()
 
    if( WALBERLA_BUILD_WITH_MPI )
-      list( INSERT  ARG_COMMAND  0  ${MPIEXEC} ${MPIEXEC_NUMPROC_FLAG} ${numProcesses} ${MPIEXEC_PREFLAGS} )
+      if( CMAKE_VERSION VERSION_LESS 3.10.0 )
+	set ( MPIEXEC_EXECUTABLE ${MPIEXEC} )
+      endif()
+      list( INSERT  ARG_COMMAND  0  ${MPIEXEC_EXECUTABLE} ${MPIEXEC_NUMPROC_FLAG} ${numProcesses} ${MPIEXEC_PREFLAGS} )
    elseif( numProcesses GREATER 1 )
       return()
    endif()