FindMPI: Make tests valid C89 by returning 0

Modules/FindMPI/libver_mpi.c

@@ -16,4 +16,5 @@ int main(int argc, char* argv[])
 #else
   puts(mpilibver_str);
 #endif
+  return 0;
 }

Modules/FindMPI/test_mpi.c

@@ -34,4 +34,5 @@ int main(int argc, char* argv[])
   MPI_Init(&argc, &argv);
   MPI_Finalize();
 #endif
+  return 0;
 }