AndrewJ, for whatever its worth, work is currently being done towards GPLing GZDoom, which is a very nice port. I think it even has experimental support for build maps.
The GPL'ing of GZDoom remains to be seen.
If it does happen, all the code from Build will have to be removed.
It also seems, if it happens, there will be the "GPL" version and the "good" version, since Graf does not want to completely remove FMOD.
Other things I don't like: Linux support in GZDoom is either missing or lousy, and many features (like models) only work in OpenGL renderer.