[boost] [BGL] Change to topo_sort_visitor breaks on GCC 2.95.3

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-05-23 11:21:00

This addition to topo_sort_visitor breaks the use of topological_sort on GCC 
2.95.3:

  template <typename Edge, typename Graph>
  void back_edge(Edge& u, Graph&) { throw not_a_dag(); }

"Edge&" should be "const Edge&" or "Edge", because there is no reason to be 
passing around edge descriptors by non-const reference.

        Doug