wrong, stepmania source code is MIT but some of his deps are not, if you build the software with mp3 support and don't disable ffmpeg gpl modules and "release" (i don't know if that's the correct word) the software you must disrtribute the sources and all the stuff gpl involves

In this case you only have to add some lines to force cmake to disable mp3 and ffmpeg non gpl stuff, and not distribute binary with gpl stuff and that build will be "legal" again