I agree that cross-platform is tricky and in the past I have used SCons with some success for that. Although, at least for Xcode, I can build my entire Xcode p

I agree that cross-platform is tricky and in the past I have used SCons with some success for that. Although, at least for Xcode, I can build my entire Xcode project from a Makefile: you just have to use command-line tools like "xcodebuild", and if you want a clickable version you set up a build phase that runs "make". And, while Apple’s ".xcconfig" files are not exactly "make" syntax, they support a subset that looks exactly the same so it is possible to have files with variable settings, etc. that can be referenced by both Xcode and any Makefile that needs to stay in sync. Environment variables are yet another option.