Now built on Half-Life Featureful SDK for GoldSource & Xash3D.
See the feature overview for Half-Life Featureful SDK on the Wiki.
Either clone the repository via git or just download ZIP via Code button on github. The first option is more preferable as it also allows you to search through the repo history, switch between branches and clone the vgui submodule.
To clone the repository with git type in Git Bash (on Windows) or in terminal (on Unix-like operating systems):
git clone --recursive https://github.com/catboxed/hl-akrasia