Don't require -DSK_USE_POSIX_THREADS.

Don't require -DSK_USE_POSIX_THREADS.

To compile SkCondVar, we already require either pthreads or Windows.  This
simplifies that code to not need SK_USE_POSIX_THREADS to be explicitly defined.
We'll just look to see if we're targeting Windows, and if not, assume pthreads.

Both before and after this CL, that code will fail to compile if we're not on
Windows and don't have pthreads.

BUG=skia:

Review URL: https://codereview.chromium.org/869443003