Fix sequential build

src/utils/Messenger.hpp

@@ -87,8 +87,9 @@ class Messenger
   static Messenger* m_instance;
   Messenger(int& argc, char* argv[]);
 
+#ifdef PUGS_HAS_MPI
   MPI_Comm m_pugs_comm_world = MPI_COMM_WORLD;
-
+#endif   // PUGS_HAS_MPI
   size_t m_rank{0};
   size_t m_size{1};