Enable the "new" V8 API for people on Mac with V8 from Homebrew

Change-Id: If7a25f0c7e91c2361ea466b3296a609beb0e8e4c
Reviewed-on: http://review.couchbase.org/30659
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Sarath Lakshman <sarathlakshman@slynux.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>