This is done by default in depth_first_search() in BGL, so we switch to
depth_first_visit().

This avoids a conflict with the overloaded tie() function.

Dumping boost::adjacency_list<> improves memory usage by a factor of
two, and also slightly improves performance in some cases.

This enables the inclusion of the non-boost tr1 headers if a recent
enough version of libstdc++ is detected. This als fixes a bug with
clang when sparsehash pulls unwanted tr1 headers.