A super simple way is to have a script in your home directory - far away from your repos - that set environment variables that you read in your configuration.
[UPDATE] I ended up doing something even simpler. I have issues with running scripts during the build process, unless really necessary (I have done it, and will, again).
Since this is Xcode, I simply needed to store the file in a directory (still with the global ignored name) far out of my dev tree, and dragged the file into the IDE.
