If you do want to make it usable for other programs, you may also want to consider using the LGPL (or something else) instead of the GPL. mcm is in a pretty crowded space with lots of competition from both proprietary and non-copyleft free software, so the GPL is probably more likely to push people to different software, instead of pushing them to release their code. I think that even the FSF would use the LGPL for something like this.
That said, if you feel it is more important that people don't use this in proprietary software than that people actually use it, GPL makes sense. I can certainly sympathize with that, and if you decided to stick with the GPL I won't complain—I just wanted to make sure you were aware of the issue.
If you do want to make it usable for other programs, you may also want to consider using the LGPL (or something else) instead of the GPL. mcm is in a pretty crowded space with lots of competition from both proprietary and non-copyleft free software, so the GPL is probably more likely to push people to different software, instead of pushing them to release their code. I think that even the FSF would use the LGPL for something like this.
That said, if you feel it is more important that people don't use this in proprietary software than that people actually use it, GPL makes sense. I can certainly sympathize with that, and if you decided to stick with the GPL I won't complain—I just wanted to make sure you were aware of the issue.