Unfortunately there are multiple other errors too when building Plastimatch with ITKv5.
I wish some more testing had been done before making the switch, at least trying to build the most downloaded extensions. Now we have to scrape to have a working SlicerRT, which is by far the most downloaded extension. I already got emails about why it is not available. This is an emergency for us that I think would have been avoidable.
For others struggling with the same thing, here is the ITK 5 migration guide: https://github.com/InsightSoftwareConsortium/ITK/blob/master/Documentation/ITK5MigrationGuide.md