Note that for every source file you can specify which target(s) it belongs to - look at the inspector window for a file (Get Info) and then hit the Targets tab. If you deselect a target for a given source file then it won't be compiled as part of the build process for that target.
[This amounts to much the same thing as what Eimantas has said in his answer - it's just a different way of looking at it.]
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…