compat/w32pthreads: use the CONDITION_VARIABLE typedef if available

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

Tested-by: Matt Oliver <protogonoi@gmail.com>
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>