Nick Mathewson d4977b52f0 Add a condition variable backend, with implementations for pthreads and win32
The interface from the user's POV is similar to the locking
implementation: either provide a structure full of function
pointers, or just call evthread_use_*_threads() and everything will
be okay.

The internal interface is meant to vaguely resemble pthread_cond_*,
which Windows people will better recognize as *ConditionVariable*.
2010-08-17 13:15:34 -04:00
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