[Halld-offline] sim-recon on Mac OS X

[David Lawrence]

Hi All,

  This is a heads up that changes to the HDDM code committed last week now require
the C++ thread_local feature. This only affects people compiling on Mac OS X. Up
until recently, the C++ compiler shipped by Apple as part of Xcode did not support
thread_local. As of Xcode 8, it now does.

What this means is that you will need to upgrade to Xcode 8 in order to compile the
latest sim-recon. If this causes any issues for anyone, please let us know.

Regards,
-David