Apple's clang (Apple LLVM version 8.0.0 (clang-800.0.24.1)) as shipped with Xcode 8 now supports `thread_local`. This effectively reverts commits: 14c6a70f3d1aba9d1d1a1a and 172ce9d.
With the boost thread helper we have to statically link to the boost libraries, which on most system is built against the latest version of the OS (instead of our current target 10.7). This doesn't seem to be an actual problem, but newer versions of clang issue warnings about it.
The new version of clang (Apple LLVM version 6.1.0) as shipped with Xcode 6.3, disabled TLS. According to http://clang.llvm.org/cxx_status.html, in order to support `thread_local`, the C++ runtime library from g++-4.8 or later is needed.
For now, we can use the boost `thread_specific_ptr`. This is probably a reasonable solution since 1) it should be portable with old and future versions of (Apple's) clang and 2) requires no additional dependencies.
This is to avoid cyclic dependencies since it was previously in a somewhat high-level framework, so everything that framework depended on, could not augment crash reports.
