[strategies][comaprable distance] add default_comparable_strategy:

this is a placeholder for comparable distance strategies used for
resolving strategies during the resolve variant stages

include/boost/geometry/strategies/default_comparable_strategy.hpp

@@ -0,0 +1,30 @@
+// Boost.Geometry (aka GGL, Generic Geometry Library)
+
+// Copyright (c) 2014, Oracle and/or its affiliates.
+
+// Contributed and/or modified by Menelaos Karavelas, on behalf of Oracle
+
+// Licensed under the Boost Software License version 1.0.
+// http://www.boost.org/users/license.html
+
+#ifndef BOOST_GEOMETRY_STRATEGIES_DEFAULT_COMPARABLE_STRATEGY_HPP
+#define BOOST_GEOMETRY_STRATEGIES_DEFAULT_COMPARABLE_STRATEGY_HPP
+
+
+namespace boost { namespace geometry
+{
+
+// This is a comparable strategy placeholder type, which is passed by
+// the algorithm free functions to the multi-stage resolving
+// process. It's resolved into an actual strategy type during the
+// resolve_strategy stage, possibly depending on the input geometry
+// type(s). This typically happens after the resolve_variant stage, as
+// it needs to be based on concrete geometry types - as opposed to
+// variant geometry types.
+
+struct default_comparable_strategy {};
+
+}} // namespace boost::geometry
+
+
+#endif // BOOST_GEOMETRY_STRATEGIES_DEFAULT_COMPARABLE_STRATEGY_HPP