See http://svn.boost.org/svn/boost/trunk r76133 "Fix threading detection in
GCC-4.7 experimental."  Without it, BOOST_DISABLE_THREADS would be defined when
compiling with GCC 4.7, which in turn would cause
boost/smart_ptr/detail/sp_counted_base.hpp to include a single-thread--mode
boost/smart_ptr/detaul/sp_counted_base_nt.hpp which is not suitable in a
multi-threaded scenario.

Change-Id: Ia30a5c2c241e2e135858a5cab3dd855d05db50c8