OS X: Use Boost.Context by default
OS X: Disable TLS by default; no Clang support yet
TLS: always #define HPX_EXPORT_THREAD_SPECIFIC_PTR
Exclude librt when targeting APPLE
HPX_AddConfigTest: Fail test when Clang prints the warning
OS X: Compile with -stdlib=libc++ and -ftemplate-depth=256
threading::no_lock: Silence unused variable warning
OS X: Don't #include <link.h> on __APPLE__.
Compile with BOOST_RESULT_OF_USE_TR1 until HPX supports decltype
OS X: Fix compiler and linking errors
Clang: Silence unused function warning
Fix a couple -Wvexing-parse warnings.

(Probably needs to be fixed in the rest of wait_any_*.hpp too!)
Merge pull request #653 from pyrtsa/osx-clang

Make HPX build on OS X + Clang + libc++