[PATCH] revised fix for nanosleep check in GLIBCXX_ENABLE_LIBSTDCXX_TIME for darwin

[Benjamin De Kosnik]

> I don't like the sched_yield macro being set there because it's
> detected correctly by configure anyway, but I'm not going to labour
> that point any more.

Indeed. Then somebody will waste hours in the future wondering why
configure says no but their TU says yes. 

At least a comment in the configure bits admitting defeat, people.

-benjamin