Probably this is pretty simple, but I can\'t find a way to define a preprocessor macro for a target in Xcode 6.
In Xcode 9 you have to add a preprocessor macros to Project, not Target. Also don't forget to add "D" as the firs letter. For example, "DDebug", then it works.
I've done a screenshot to show where it is in Xcode, because it's easier :)