w32pthreads: use the CONDITION_VARIABLE typedef if available

This silences warnings about passing arguments from incompatible pointer type
when targeting Windows Vista or newer.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Martin Storsjö <martin@martin.st>
w32pthreads: use the condition variable API directly when targeting newer versions of Windows

Wrap the function calls in a similar fashion to how it's being done
with the critical section API.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Martin Storsjö <martin@martin.st>