Sort out so that can still compile with only METIS

src/debug.c

@@ -393,7 +393,7 @@ void dumpCells(const char *prefix, int active, int mpiactive, int pactive,
   fclose(file);
 }
 
-#ifdef HAVE_METIS
+#if defined(WITH_MPI) && defined(HAVE_METIS)
 
 /**
  * @brief Dump the METIS graph in standard format, simple format and weights

src/debug.h

@@ -39,7 +39,7 @@ int checkCellhdxmax(const struct cell *c, int *depth);
 void dumpCells(const char *prefix, int active, int mpiactive, int pactive,
                struct space *s, int rank, int step);
 
-#ifdef HAVE_METIS
+#if defined(WITH_MPI) && defined(HAVE_METIS)
 #include "metis.h"
 void dumpMETISGraph(const char *prefix, idx_t nvtxs, idx_t ncon, idx_t *xadj,
                     idx_t *adjncy, idx_t *vwgt, idx_t *vsize, idx_t *adjwgt);