Add feature test for thread_local on Clang for TLS

[zao]

Clang has __has_feature which can be used to query for whether
thread_local exists. As clang with libc++ isn't covered by the
existing thread_specific_ptr cases, this adds support for TLS
on this combination of compiler and runtime.

This should resolve the issues in issue #1880.

[hkaiser]

Is there really a guarantee that any system implementing thread_local also exposes __has_feature(cxx_thread_local)?

[zao]

The existing fallback cases will still be there for implementations that do lack __has_feature or the feature cxx_thread_local.

The documentation I consulted for this change is http://llvm.org/releases/3.3/tools/clang/docs/LanguageExtensions.html#c-11-thread-local which claims that if the feature is present, the Clang compiler in the current configuration implements thread_local.

As for widespread compiler support for __has_feature, I'm not aware of any compiler that implements the Clang-sourced feature detection macro. GCC has a feature request open since 2014 but seem to have been sidetracked by the SD6 initiative.

[hkaiser]

LGTM, thanks!