[boost] Proposal: Add build output types to Boost.Config

If there is not currently a set of macros provided by config.hpp which detect the type of output the compiler is generating, is there interest in adding one? I am thinking something along the lines of: BOOST_CAN_DETECT_BUILDING_EXE BOOST_BUILDING_EXE BOOST_CAN_DETECT_BUILDING_SLL BOOST_BUILDING_SLL BOOST_CAN_DETECT_BUILDING_DLL BOOST_BUILDING_DLL -Jason