[patch] fix thread test cases for solaris.

[Paolo Carlini]

.. in fact, I have a much simpler solution (I'm fixing the logic my
first try :)

Both in case of yes and rt, we run:

  AC_SEARCH_LIBS(sched_yield, [rt posix4])

Then, if either  we find nothing, or we find the function in librt and
the user allowed for it, we define _GLIBCXX_USE_SCHED_YIELD; otherwise
we don't define it.

It's a bit delicate, but I think covers all the cases we know.

Paolo.