I'd like to drop support for Apple Clang. With the C++14 upgrade, we'll be requiring many distros to fetch a newer compiler. In most cases it only takes a few commands to get a newer compiler. This is also true of OS X, where clang-4.0 can be easily installed with `brew install llvm`.
The current codebase does not compile with Apple Clang under C++14 mode. We could choose to investigate whether this is a Mesos bug or an Apple Clang bug, but after doing a brief investigation myself, I feel like it's not worth the effort. There are already cases where we need to install a new compiler on OS X due to Apple Clang releases based on clang-3.8 (MESOS-5745 <https://issues.apache.org/jira/browse/MESOS-5745>). Not that Apple Clang was "officially" supported anyway, but we have had minor workarounds (e.g., THREAD_LOCAL) to support it. Please let me know what you think! Thanks, MPark
