Without this, files are added to the exclude list on all configurations,
which disables autocompletion on Xcode. If a file is just excluded in
some configurations it's handled the old way.
As discussed numerous times, get rid of the submodule approach for core modules and just included the sources directly in the main repository.
I have no idea how this will play with existing working copies. It might be necessary to [manually clear out the existing submodules](http://stackoverflow.com/questions/1260748) first.
