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.
While reading the ```__APPLE__``` define I wondered why we have an ```MAC_OS``` define for OS X systems (disregarding, that ```MAC_OS``` is an unfavorable name).
Is ```__APPLE__``` standardized or widely used (e.g. in all our compilers we want to support: clang from xcode 5 and xcode 6) so that it is safe to replace all occurrences of ```MAC_OS``` with ```__APPLE__``` in all of our projects?