message: Remove unused variable warning when MPI is disabled.
diff: int main(int argc, char *argv[])
  {
  int rank = 0;
  - int size = 1;
  #ifdef ADIOS2_HAVE_MPI
  + int size = 1;
  MPI_Init(&argc, &argv);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);
  