We are currently working on creating a new simpleitk.org website, unfortunately the old one is a little out of date.
The best documentation on how to get started building SimpleITK is here: https://simpleitk.readthedocs.io/en/master/building.html#
The compilation errors you are receiving is because you are compiled a version of SimpleITK that worked with ITKv4 and does not support the changes in ITKv5. Those changes are currently in the SimpleITK master branch, and the next release will support the stabalized ITK 5.1.
In the linked instructions, it strongly suggests using SimpleITK’s Superbuild, which automatically builds the proper version of ITK ( and other requirements ).