Recompiling ITK after compiling ITKVtkGlue

I am getting the following error message when trying to recompile ITK after having compiled ITKVtkGlue as a separate project:

```
/home/francois.budin/devel/itk/ITK/build/Wrapping/Modules/ITKVTK/itkVTKImageImportPython.cpp:4609:10: fatal error: vtkImageImport.h: No such file or directory
#include "vtkImageImport.h"
^~~~~~~~~~~~~~~~~~
compilation terminated.
Wrapping/Modules/ITKVTK/CMakeFiles/ITKVTKPython.dir/build.make:295: recipe for target 'Wrapping/Modules/ITKVTK/CMakeFiles/ITKVTKPython.dir/itkVTKImageImportPython.cpp.o' failed
make[2]: *** [Wrapping/Modules/ITKVTK/CMakeFiles/ITKVTKPython.dir/itkVTKImageImportPython.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
/home/francois.budin/devel/itk/ITK/build/Wrapping/Modules/ITKVTK/itkVTKImageExportPython.cpp:4566:10: fatal error: vtkImageImport.h: No such file or directory
#include "vtkImageImport.h"
^~~~~~~~~~~~~~~~~~
compilation terminated.

```

This could be related to the fact that ITKVtkGlue is both inside and outside the ITK repository.