Is there a reason you are trying to build Slicer-4.8 instead latest master? We cannot spend much time with helping developers trying to make very old Slicer versions work.
In this field, 3 years is a long time. Instead of struggling with trying to review an old software stack, trying to nag application and library developers with supporting old code, and in the end producing something that most people cannot use (since nobody uses Slicer-4.8 anymore), I would recommend to skip forward and build the extension with the latest Slicer version. Lots of things have changed in ITK, VTK, Slicer, Python, etc. but you can always get help for updating software to work with latest application/library versions. If you run into any trouble with Slicer we can definitely help here, and probably with most VTK, ITK, Python problems, too.
Got it. This solution was a long-term plan, to update the extension, but I will change my plans and start working on it. Thanks for the attention @lassoan, for sure I will need more help in the near future