[patch] fix thread test cases for solaris.

[Paolo Carlini]

Hi,

> testsuite/30_threads/thread/this_thread/2.cc uses
> std::this_thread::yield(); which is based on __gthread_yield() which
> in turn is just a sched_yield().

Ok, I also realized that, in the meanwhile. Apparently, we didn't  
anticipate that gthr functions could be implemented in librt...

> We have --enable-clock-gettime=rt to solve this for clock_gettime,
> perhaps we a --enable-nanosleep=rt, --enable-sched_yield=rt or
> something more general

Well, if librt can be used all those basic facilities are likely to be  
available. I would ask you to figure out a way to have a single large  
test, starting from the existing one for clock-gettime, renamed.  
Inside, link tests for each of the three facilities, not run by  
default, etc, exactly the same we do for clock-gettime. Should not be  
too difficult to implement...

Paolo