For CentOS, install the InsightToolkit-devel and InsightToolkit-vtk-devel (if available) packages to make the C++ headers and CMake build configuration files available. The InsightToolkit package only contains runtime libraries, etc.
Thank you very much for your reply. I downloaded, and installed, itk-devel-3.4-9.el7.x86_64.rpm using
sudo yum install itk-devel-3.4-9.el7.x86_64.rpm
Unfortunately,
sudo find / -name ITKConfig.cmake -print
returns no results.
I tried to find vtk-devel for CentOS. I found links but they only took me to more links. I was unable to find an rpm file to download. Eventually, I found that I was going in a circle.