As you can see, there is an Apache Ant build.xml in the repo. I cloned the repo, and downloaded the forge src zip file. I unzipped this zip file into the Minecraft Forge directory which you can see is referenced in the build.xml. I then ran ant to build the project - after running install.cmd in the downloaded forge environment. It seemed to work - it outputted the correct JARs into the output directory, however there were no .class files in the JAR. The only thing that was put into the JAR files were the resources.
I looked at the reobf folder in the copied over mcp folder, and it had a minecraft folder which was empty - I think this may be related to the issue but am not sure.
Any and all help would be very appreciated - I would be glad to provide more information if needed.