Something like Spirit of Half-Life. The intent is to add more features to the Half-Life SDK while better documenting the engine's features and maintaining the same style.
$ # clone the repo
$ git clone --mirror https://github.com/tschumann/basis
$ cd basis
$ # push the checkout of Basis into a repo that you have created
$ git push --mirror https://github.com/you/basis-mirror
$ # clone your repo
$ git clone https://github.com/you/repo
$ # note that similar steps are in update.bat too
$ # create a new remote that points to Basis
$ git remote add basis https://github.com/you/basis-mirror
$ # pull and merge the latest changes from Basis
$ git pull basis master
$ sudo apt-get install gcc-multilib g++-multilib mesa-common-dev
$ cd linux
$ make
$ # clone the repo
$ git clone https://github.com/tschumann/basis
$ # create a new remote to link back to Valve's halflife repo
$ git remote add valve https://github.com/ValveSoftware/halflife
$ # pull the latest changes from halflife
$ git pull valve master