Conditionally compile MPI calls in `config_graph`

Because we want to call `config_graph` in sequential program
`test_overlap`.

Overlap/config_graph.F90

@@ -21,7 +21,9 @@ contains
     use, intrinsic:: ISO_FORTRAN_ENV
 
     ! Libraries:
+#ifndef SEQUENTIAL
     use ezmpi, only: ezmpi_bcast
+#endif
     use jumble, only: assert, pi
 
     use read_grid_m, only: periodic, step
@@ -45,10 +47,12 @@ contains
        copy = merge(dist_lim, 0, periodic)
     end if
 
+#ifndef SEQUENTIAL
     call ezmpi_bcast(max_delta, root = 0)
     call ezmpi_bcast(dist_lim, root = 0)
     call ezmpi_bcast(cyclone, root = 0)
     call ezmpi_bcast(copy, root = 0)
+#endif
 
   end subroutine config_graph