Alright, it wasn't that hard. I just cloned the source code from a mirror, `make`'d and `make install`'ed:
1. Clone Git repository of `afl`, I use a mirror I found on GitHub:
git clone
2. Change directory and `make` and `make install`:
cd afl
make && sudo make install
Of course, there might be some libraries you need to install in order to compile. I did not need to do this.