message: Fix the serial build with SUNDIALS.
diff: @@ -155,12 +155,16 @@ void CVODESolver::Init(TimeDependentOperator &f_)
  // Get the vector length
  long local_size = f_.Height();
  +#ifdef MFEM_USE_MPI
  long global_size;
  +#endif
  if (Parallel())
  {
  +#ifdef MFEM_USE_MPI
  MPI_Allreduce(&local_size, &global_size, 1, MPI_LONG, MPI_SUM,
  NV_COMM_P(y));
  +#endif
  }
  // Get current time
  @@ -169,7 +173,7 @@ void CVODESolver::Init(TimeDependentOperator &f_)
  if (sundials_mem)
  {
  // Check if the problem size has changed since the last Init() call
  - int resize;
  + int resize = 0;
  if (!Parallel())
  {
  resize = (NV_LENGTH_S(y) != local_size);
  @@ -565,12 +569,16 @@ void ARKStepSolver::Init(TimeDependentOperator &f_)
  // Get the vector length
  long local_size = f_.Height();
  +#ifdef MFEM_USE_MPI
  long global_size;
  +#endif
  if (Parallel())
  {
  +#ifdef MFEM_USE_MPI
  MPI_Allreduce(&local_size, &global_size, 1, MPI_LONG, MPI_SUM,
  NV_COMM_P(y));
  +#endif
  }
  // Get current time
  @@ -579,7 +587,7 @@ void ARKStepSolver::Init(TimeDependentOperator &f_)
  if (sundials_mem)
  {
  // Check if the problem size has changed since the last Init() call
  - int resize;
  + int resize = 0;
  if (!Parallel())
  {
  resize = (NV_LENGTH_S(y) != local_size);
  @@ -1049,18 +1057,22 @@ void KINSolver::SetOperator(const Operator &op)
  // Get the vector length
  long local_size = height;
  +#ifdef MFEM_USE_MPI
  long global_size;
  +#endif
  if (Parallel())
  {
  +#ifdef MFEM_USE_MPI
  MPI_Allreduce(&local_size, &global_size, 1, MPI_LONG, MPI_SUM,
  NV_COMM_P(y));
  +#endif
  }
  if (sundials_mem)
  {
  // Check if the problem size has changed since the last SetOperator call
  - int resize;
  + int resize = 0;
  if (!Parallel())
  {
  resize = (NV_LENGTH_S(y) != local_size);
  