The "Devon" folder inside "Source" is an Eclipse project which means you're able to compile it in Eclipse.
I never really got into studying and using makefiles, so I really don't know how to make one :/
I can't really create the jar file because I've worked on the game after releasing it :S
If you install Eclipse and open the project you'll be able to compile it though (to run the game from inside eclipse, to debug it, you need to add the data folder as a class folder and go through all the files and change all the strings that are paths to files and remove "/data", this because the executable jar I compiled to release wasn't working right with the resources being inside the file).