I like very much the idea of having vtkAddon as a separated project as introduced in aaf75e2aaa. In this logic, one can think about vtkAddon being a dependency of Slicer --this helps e.g., supporting the idea of having 3D Slicer installed in a system alongside other related projects, where the common dependencies are shared.
In the current implementation, the python wrapping of vtkAddon relies on macros defined in vtkMacroKitPythonWrap.cmake (which is part of 3D Slicer), making impossible to build vtkAddon by itself and producing a cyclic dependency vtkAddon–>Slicer–>vtkAddon.
I would suggest to move a copy of vtkMacroKitPythonWrap.cmake to vtkAddon/CMake as a temporary solution, assuming that the integration of the official VTK will soon come, and that VTK will be the provider of the wrapping functions. Does it make sense? Any thoughts?