ifdef HAVE_MPI to remove warnings

Solver/linearSystemPETSc.hpp

-#ifdef HAVE_PETSC
+#if defined(HAVE_PETSC)
+
 #include <petsc.h>
 #include <petscksp.h>
 #include "linearSystemPETSc.h"
 
-
 static void  _try(int ierr)
 {
   CHKERRABORT(PETSC_COMM_WORLD, ierr);
@@ -79,6 +79,7 @@ linearSystemPETSc<scalar>::~linearSystemPETSc()
 template <class scalar>
 void linearSystemPETSc<scalar>::_assembleMatrixIfNeeded()
 {
+#if defined(HAVE_MPI)
   if (_comm == PETSC_COMM_WORLD){
     if (Msg::GetCommSize() > 1){
       int value = _valuesNotAssembled ? 1 : 0;
@@ -89,7 +90,7 @@ void linearSystemPETSc<scalar>::_assembleMatrixIfNeeded()
       }
     }
   }
-
+#endif
   if (_valuesNotAssembled) {
     _try(MatAssemblyBegin(_a, MAT_FINAL_ASSEMBLY));
     _try(MatAssemblyEnd(_a, MAT_FINAL_ASSEMBLY));
@@ -179,7 +180,7 @@ void linearSystemPETSc<scalar>::allocate(int nbRows)
   _try(MatSetFromOptions(_a));
   //since PETSc 3.3 GetOwnershipRange and MatGetSize cannot be called before MatXXXSetPreallocation
   _localSize = nbRows;
-  #ifdef HAVE_MPI
+#if defined(HAVE_MPI)
   if (commSize > 1){
     _localRowStart = 0;
     if (Msg::GetCommRank() != 0) {
@@ -327,6 +328,7 @@ void linearSystemPETSc<scalar>::getFromSolution(int row, scalar &val) const
 template <class scalar>
 void linearSystemPETSc<scalar>::zeroMatrix()
 {
+#if defined(HAVE_MPI)
   if (_comm == PETSC_COMM_WORLD){
     if (Msg::GetCommSize() > 1){
       int value = _entriesPreAllocated ? 1 : 0;
@@ -337,6 +339,7 @@ void linearSystemPETSc<scalar>::zeroMatrix()
       }
     }
   }
+#endif
   if (_isAllocated && _entriesPreAllocated) {
     _assembleMatrixIfNeeded();
     _try(MatZeroEntries(_a));