[patch] fix thread test cases for solaris.

[Paolo Carlini]

Hi,
> So define _GLIBCXX_USE_SCHED_YIELD based on $ac_has_sched_h basically?
>   
I don't see why we should check anything having to do with sched.h or
similar system headers. I think only ac_cv_search_sched_yield matters,
because in that case we know for sure that sched_yield is implemented in
librt. Agreed, in principle __gthread_yield could not use sched_yield,
but checking sched.h doesn't help about that, and I don't think we are
taking a big risk, because the user allowed for librt anyway, and I
think it's unlikely librt is not necessary for clock_gettime.

Paolo.