Created attachment 197952 [details]
Per upstream (http://vtk.1045678.n5.nabble.com/Problem-with-building-VTK-td5742905.html), building using GCC 6 and newer is not supported, they advise using GCC 5. Use that to build this port on architectures using GCC, since build fails with GCC 7.
While here, add USES=gl.
Tested on powerpc64.
Hardware sponsored by IntegriCloud.
The port does not build, see https://svnweb.freebsd.org/ports?view=revision&revision=483941 (verified today on 11.2-RELEASE amd64).
Remove this port instead.