Fix OSX support for new CLI Tools (starting with 7.1)

To build on OSX with late-2015 CLI Tools, the build system must include C++
headers from a new location.  Mid-2015 CLI Tools and older can still be used, so
the older location will also be kept.

Change-Id: I422d28cb41ab57fcc816538a822df8aecdf7d9d6