Fix Xcode deprecation warning for OSMemoryBarrier()

[jdonald]

Fix this warning in autoboost :

autowiring/contrib/autoboost/autoboost/asio/detail/macos_fenced_block.hpp:51:5: warning: 
      'OSMemoryBarrier' is deprecated: first deprecated in macOS 10.12 - Use
      std::atomic_thread_fence() from <atomic> instead
      [-Wdeprecated-declarations]
    OSMemoryBarrier();

Doesn't seem to be fixed in the boost repo yet so I couldn't mimic something there.

On github I found in the repo for cmus, a music player, they used std::memory_order_seq_cst. The documentation seems to indicate OSMemoryBarrier() is a sequential consistency barrier too.