GCC, LLVM, and compiler plugins

Right, my mistake. Still, this only changes my argument slightly: a copyright license can still demand anything in return for giving you permission to copy or distribute. It may demand that you don't link it with proprietary software just like it may demand that you wear purple pajamas while doing the distributing.

This does restrict the power of the GPL: it cannot say anything about linking the program with a proprietary one on you own system. But it can say just fine that it considers such linking a BadThing, and it may not be distributed while it is a BadThing.