Fix build on Mac OS X 10.7 Lion.

`CVDisplayLinkRef` needs an extra header file, and `NSAppKitVersionNumber10_7`
wasn't added until 10.8so use `> NSAppKitVersionNumber10_6` instead.
Merge branch 'fix-10-7-build' into fix-10.7
Merge pull request #2 from metajack/fix-10.7

Fix the build on OS X 10.7