Is there a place (or flag) in Xcode for files that you don't want to compile? There are some classes that are/may become part of a project but currently won't compile. The main project doesn't link to them but Xcode still tries to compile them. Is there a way to prevent blocking the rest of project from compiling until these new Classes are "ready"?
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.]