I dislike adding a function which always return 0 when the feature is not supported:

unsigned long
PyThread_get_thread_native_id(void)
{
    ...
#if ...
    ...
#else
    unsigned long native_id;
    native_id = 0;
#endif
    return (unsigned long) native_id;
}

I would prefer to not define the function if it's not available, so the attribute would be missing.

With the commited change, how can I know if native thread identifier is supported or not?