[boost] [config] request for BOOST_NO_FWD_STD_DECLARATION

libc++ does not allow forward declarations of types for std:: , as it uses inline namespace to allow versioning. Forward declarations occur in many libraries (config, fusion, interprocess). In each case I would like to just include the actual headers. I could just #define all cases for LIBCPP, but other libraries might move to using inline namespaces in future, so I could introduce a macro? Chris