Select the proper overload for boost::bind.

In C++11 there are two viable overload of the function. Specify the
proper overload to avoid a compilation error.

src/unit.cpp

@@ -434,7 +434,13 @@ unit::unit(const config &cfg, bool use_traits, const vconfig* vcfg) :
 	{
 		cfg_.clear_children("advancement");
 		boost::copy( cfg.child_range("advancement")
-			, boost::make_function_output_iterator(boost::bind( &config::add_child, boost::ref(cfg_) /*thisptr*/, "advancement", _1 )) );
+			, boost::make_function_output_iterator(
+				boost::bind(
+					  /* The static_cast is required to select the proper overload in C++11 mode. */
+					  static_cast<config&(config::*)(const std::string &, const config&)>(&config::add_child)
+					, boost::ref(cfg_) /*thisptr*/
+					, "advancement"
+					, _1 )) );
 	}
 
 	//don't use the unit_type's abilities if this config has its own defined