Add a pthreads fallback for thread_local

[kwasimensah]

Some platforms don't properly support thread_local (android/ndk#8). So this adds a pthread based work around. It still defaults to using thread_local.

[ta0kira]

As discussed offline, we're going to hold off on this and limit support to compilers that properly implement C++11 thread_local.

Summary of rationale:

• There is no simple way to support non-compliant compilers without either complicating the implementation for compliant compilers or branching the sources.
• Branching the sources (even with #ifdef gating) would be problematic because of the thread_local in template code. This is because it would potentially require conditional build logic in multiple places in the user's code.
• At this point, I don't want to add a custom thread-local implementation or an equivalent dependency.