[boost] [config] BOOST_DISABLE_THREADS defined by Boost.Config

Joe Gottman quite correctly points out in http://svn.boost.org/trac/boost/ticket/1762 that shared_ptr should honor user definitions of BOOST_DISABLE_THREADS and disable MT support. Unfortunately, despite documenting quite clearly that BOOST_DISABLE_THREADS is a user macro, Boost.Config defines it in some situations. This means that I can't rely on this macro to reflect the user's intent, as I don't know whether it's been set explicitly by the user, or implicitly (and incorrectly IMO) by Boost.Config.