How to get support for thread_local on Mac OSX clang?

As shown in this answer, clang from Xcode on Mac OSX does not support thread_local storage even with C++11 flags set. Even on the latest version, Apple LLVM version 7.0.0 (clang-700.1.76), Target: x86_64-apple-darwin15.0.0, Thread model: posix, does not support thread_local:

../../src/dir/sysArch.h:1505:3: error: thread-local storage is not supported
                                       for the current target
  thread_local
  ^