MPI and affinity control are not compatible

MPI libraries offer affinity control so as a first guess we shouldn't attempt any control, this is really bad when running more than one rank on the same machine (threads pinned to same cores).

examples/main.c

@@ -116,6 +116,7 @@ int main(int argc, char *argv[]) {
   /* Greeting message */
   if (myrank == 0) greetings();
 
+#ifndef WITH_MPI
 #if defined(HAVE_SETAFFINITY) && defined(HAVE_LIBNUMA) && defined(_GNU_SOURCE)
   /* Ensure the NUMA node on which we initialise (first touch) everything
    * doesn't change before engine_init allocates NUMA-local workers. Otherwise,
@@ -129,6 +130,7 @@ int main(int argc, char *argv[]) {
   } else {
     message("set entry thread's affinity");
   }
+#endif
 #endif
 
   /* Init the space. */