In Linux, you have three choices of how to obtain ITK. In fact, one of the options allows you to skip Step 2: Building entirely!
Option 1: (NOT YET AVAILABLE FOR ITK4!): Download ITK from your package manager (e.g. apt-get on Ubuntu, yum on Fedora, etc)
Option 2: Download a release version of the source code. A release version has been marked as "approved and working" by the community. Simply download and unzip the zip or tar (e.g. InsightToolkit-4.1.0.tar.gz) on the download page and unzip it. You now have the ITK source code!
Option 3: Download the very latest version of the source code.