round OS X targetOS property off

The complete value for OS X is ["osx", "darwin", "bsd4", "bsd", "unix"]

Change-Id: Ib0f08e1dfb4ef2ce6e23eeffbfa054fe01b629cf
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>