[v3] mutex, condition_variable

[Benjamin Kosnik]

> The problem is different actually.  The 30_threads tests aren't linked
> with -lpthread, which means !__gthread_active_p ().  

Yeah. I forgot I had link/run tests in this version, and assumed it was
all compile-only. Whoops.

It is easy enough to just add -pthreads and variants for the run tests,
like the existing thread directory. However, I am hoping there is a
better way to do this then the present platform-specific way.

Any ideas?

> And the try_lock/2.cc testcase is buggy of course, when the
> mutex isn't recursive, try_lock on an already locked mutex is
> supposed to fail.

Will fix, should be !b in the VERIFY statement below the try_lock call.

-benjamin