Re: [boost] BOOST_HAS_PTHREADS, BOOST_HAS_SCHED_YIELD tests in config_test not strict enough

The tests for sched_yield and nanosleep now fail as expected on g++/Sun. :-) clock_gettime also fails to link across the board.

I think that we ought to fix this by adding -lrt even in <threading>single instead of #undef-ing the feature macros, since these calls aren't technically tied to threads.

How about this one?