message: Guard parallel calls in serial build
diff: @@ -306,7 +306,11 @@ void OperatorHandle::EliminateRows(const Array<int> &ess_dof_list)
  {
  case Operator::Hypre_ParCSR:
  {
  +#ifdef MFEM_USE_MPI
  this->As<HypreParMatrix>()->EliminateRows(ess_dof_list);
  +#else
  + MFEM_ABORT("type id = Hypre_ParCSR requires MFEM_USE_MPI");
  +#endif
  break;
  }
  default:
  @@ -320,8 +324,12 @@ void OperatorHandle::EliminateCols(const Array<int> &ess_dof_list)
  {
  case Operator::Hypre_ParCSR:
  {
  +#ifdef MFEM_USE_MPI
  auto Ae = this->As<HypreParMatrix>()->EliminateCols(ess_dof_list);
  delete Ae;
  +#else
  + MFEM_ABORT("type id = Hypre_ParCSR requires MFEM_USE_MPI");
  +#endif
  break;
  }
  default:
  