Define MPI_COMM_NULL when not using MPI.

diff --git a/include/deal.II/base/mpi.h b/include/deal.II/base/mpi.h
index 43177039579dfbc5e43b013e9b8973b7e3ba78ee..68e2a96187571f6253d4cb4b425762d41676101c 100644 (file)
--- a/include/deal.II/base/mpi.h
+++ b/include/deal.II/base/mpi.h
@@ -44,6 +44,9 @@ using MPI_Op       = int;
 #  ifndef MPI_COMM_SELF
 #    define MPI_COMM_SELF 0
 #  endif
+#  ifndef MPI_COMM_NULL
+#    define MPI_COMM_NULL 0
+#  endif
 #  ifndef MPI_REQUEST_NULL
 #    define MPI_REQUEST_NULL 0
 #  endif