implement setSwapInterval() for OS X

hey. this pull request adds the ability to change the swap interval on OS X in the cubescape-qt example. Adaptive vertical synchronization is not supported and results in a normal vertical synchronization.
`__APPLE__` seems to be the way to go: http://stackoverflow.com/questions/2166483/which-macro-to-wrap-mac-os-x-specific-code-in-c-c