This reduces the call overhead slightly. More noticeably, it
restores the earlier (unintended?) feature that condition variable
functions work just fine even if w32thread_init() hasn't been called.
This was broken as a side effect of 4622f11f, if explicitly targeting
Vista+.

This makes w32threading work in VP8 again, if targeting Vista+.
Signed-off-by: Martin Storsjö <martin@martin.st>