use MPI_THREAD_MULTIPLE and warn user if it is not supported

src/drivers/mpi/driver_mpi_common.c

@@ -54,9 +54,16 @@ static void _starpu_mpi_set_src_node_id()
 
 int _starpu_mpi_common_mp_init()
 {
-	if (mpi_initialized || MPI_Init(_starpu_get_argc(), _starpu_get_argv()) != MPI_SUCCESS)
+    int thread_support;
+	if (mpi_initialized || MPI_Init_thread(_starpu_get_argc(), _starpu_get_argv(), MPI_THREAD_MULTIPLE, &thread_support) != MPI_SUCCESS)
         return 0;
 
+    if (thread_support != MPI_THREAD_MULTIPLE)
+    {
+        fprintf(stderr, "MPI doesn't support MPI_THREAD_MULTIPLE option. MPI Master-Slave can have problems if multiple slaves are launched. \n");
+    }
+
+
 	mpi_initialized = 1;
 
     /* Find which node is the master */