[boost] [config][type_traits] Support for final keyword in Boost?

I have a couple of pull requests to add support for the final keyword in Boost.Config and an is_final trait to type traits, so my questions are: 1) Are these generally useful? 2) Do we need BOOST_NO_CXX11_FINAL_CLASS and BOOST_NO_CXX11_FINAL_OVERRIDE or will a single BOOST_NO_CXX11_FINAL do? 3) The trait appears to be unimplementable via SFINAE (anyone care to prove me wrong?) which means it's only usable on GCC and lookalikes which have an __is_final intrinsic. Should we still add this to Boost or is the answer "use GCC's native features" given that a Boost version effectively adds nothing? Thanks! John.