We assume that for Apple platforms that the __thread keyword doesn't work.

https://github.com/mongodb/mongo/blob/master/src/mongo/util/concurrency/threadlocal.h#L120

However, this isn't a platform thing, it is a compiler thing. Clang on Apple, for instance, does support __thread.

The check for __thread support should be moved into a Configure time check and a config.h style header, setting something like MONGO_HAVE___THREAD.