Fix a_star to work with boost 1.49
Fix for boost <1.52

Prior to boost 1.52 there was an error in the named parameter version
of astar_search_no_init. Details can be found in the boost bugtracker:
https://svn.boost.org/trac/boost/ticket/7380

As consequence the non-named parameter interface has to be used until
boost 1.52 or higher is used by the server.