Fix C++ on Darwin

We've been including the system libc++ headers even if we're building
against our version of libc++. Stop doing that, and only add the headers
to our path if we're using the system libraries.

If nothing is specified, on recent OSX versions, libc++ is the default
c++ library instead of libstdc++. We've been explicitly including the
libc++ headers on all versions, but that breaks old versions. Force us
over to libc++, since the system libstdc++ does not support C++11, and
libc++ is still supported on our oldest version (10.8).

Change-Id: I1fccee8da0f425e10ccc9d3247ed40664eb6ada0