title: Adding a Preprocessor Definition via a Xcode Scheme
body: <p>I currently have a number of special "flavors" of an iPhone application that I need to build. Ideally, I would like to have a scheme for each "flavor" and each scheme would define (or set) one or more preprocessor definitions that I can use to branch on in my code and possibly even preprocess my info.plist file. This can obviously be done with multiple targets, but since I could have many different "flavors" of the app, it would be great to do it with schemes to keep the target count down. My current thought is to add these preprocessor definitions during a pre-action script, but I can not for the life of me find any way to update GCC_PREPROCESSOR_DEFINITIONS. Since it is an environment variable, shouldn't I have access to append onto GCC_PREPROCESSOR_DEFINITIONS? </p>