Binaries are available for download in the download area but if you want, you can get the source directly from the git repository. I have put together a little guide below.
I have built and developed the game on both Ubuntu (10.10) and Gentoo. To build the project on those two systems you need Git to download and CMake to build the source. You can find them in the packet manager of your system.
Download the source
git clone git://subspacebattle.git.sourceforge.net/ gitroot/subspacebattle/subspacebattle
Install the dependencies
We need to add some extra packages to be able to build the source.
sudo emerge libsdl sdl-net sdl-sound sdl-mixer sdl-image irrlicht
Run the commands in the "trunk" directory:
cmake . && make