Fix petsc configuration when pugs is built without mpi support

By now the choice is made that mpi is required when PETSc. This may change in the future: it is not that important but would probably require the use of #ifdef directives...

CMakeLists.txt

@@ -156,8 +156,14 @@ set(PUGS_ENABLE_PETSC AUTO CACHE STRING
   "Choose one of: AUTO ON OFF")
 
 if (PUGS_ENABLE_PETSC MATCHES "^(AUTO|ON)$")
-  pkg_check_modules(PETSC PETSc)
-
+  if (MPI_FOUND)
+    # PETSc support is deactivated if MPI is not found
+    pkg_check_modules(PETSC PETSc)
+  else()
+    message(STATUS "PETSc support is deactivated since pugs will not be build with MPI support")
+    set(PETSC_FOUND FALSE)
+    unset(PUGS_HAS_PETSC)
+  endif()
   set(PUGS_HAS_PETSC ${PETSC_FOUND})
 else()
   unset(PUGS_HAS_PETSC)