XCode < 8 does not support the C++11 `thread_local` construct. As a result, we added a workaround to use `__thread` on OSX and `thread_local` on other platforms:
https://reviews.apache.org/r/36845/ Since that workaround was added, XCode 8 has been released (in September 2016) with support for `thread_local`. If we required XCode >= 8.0 for building Mesos, we could remove this workaround and use `thread_local` on all platforms. This would have some advantages, such as: (1) More uniform, standards-compliant code between OSX and other platforms. (2) `thread_local` supports a few features that `__thread` does not, such as support for non-POD data types. Therefore, I'd like to propose we make XCode 8 a requirement for building Mesos on OSX, and then get rid of the `THREAD_LOCAL` workaround. Since XCode 8.0 requires OSX 10.11.5, this would also imply dropping support for OSX 10.10. Feedback welcome. Neil
