Need to have a virtual destructor since we have one virtual function

(g++ 4.0 complains).  This function was commented out from many years
ago in the "bad old days" of new-not-conformant compilers.

This commit was SVN r6038.

src/mpi/cxx/comm.h

@@ -62,7 +62,7 @@ public:
   inline Comm_Null(const MPI_Comm& data) : mpi_comm(data) { }
 
   // destruction
-  //JGS virtual inline ~Comm_Null() { }
+  virtual inline ~Comm_Null() { }
 
  // comparison
   // JGS make sure this is right (in other classes too)