Josh, thank you for your honest and direct reply. I can finally see how unlikely it is that you will compromise your beliefs regarding ENIGMA's engine license. I am firmly opposed to any sort of custom license for ENIGMA's code, but I do agree that a custom license is the only way to reach your goal of "[forbidding] ENIGMA clones which forbid ENIGMA clones." Although I don't agree with your need for a custom license, I wish you the best of luck.
I'm feeling nostalgic at the moment and I'd like to explain why I went looking for a project like ENIGMA. I don't remember exactly when, but I found a game called
Spelunky many years ago. I'm sure that most of you have heard of it and/or played it. If not, you should stop reading this and check it out immediately because it's a great game! Spelunky is made with GameMaker and its author made the source code available to everyone. I wanted to play this game on Linux, but it is a Windows only game. With the code being available and not wanting to use Wine or a virtual machine to play it, I tried to find a way to port it to Linux. This was years before GameMaker Studio existed, but it's not like I can afford GameMaker Studio or it's Linux export plugin anyway. I searched for a while and finally stumbled onto ENIGMA.
At the time, ENIGMA lacked too many features and I would have had to change too much of Spelunky's code to get it working. I decided to put the idea aside and wait for ENIGMA to improve. Years later, I saw that there was a Mac OS X port of Spelunky and I decided to revisit porting the game to Linux. ENIGMA was in much better shape and seemed up to the task. There was just one problem, Spelunky has a custom license created by it's author. This license is incompatible with the GPLv3 (I swear that I'm not preaching about the perils of custom licenses). While reading through the forums here, I saw that ENIGMA was having licensing issues of it's own.
I (foolishly) thought that this was an opportunity to help an open source project and my Linux port of Spelunky at the same time. "If I can convince ENIGMA's developers to switch to a license that allows any type of code to link to the engine, then everybody wins!" It seems that both of my goals have been destined for failure. Oh well, I tried my best, but it's time to move on.