Only activate MacOS X compilation by default on 10.10 (Yosemite) or higher.

Older revisions of MacOS X will use the generic Unix mode by default, since
the `vmnet` framework requires Yosemite or higher.

Reported by: Brian Brietzke <bbrietzke@gmail.com>
Merge pull request #337 from avsm/mirage-autodetect-osx

Only activate MacOS X compilation by default on 10.10 (Yosemite)