But if I change the run-time library, I still have to license those changes under the GPLv3 if I want to distribute them, right?
Yes. But if you change the runtime library and link something else with the modified runtime library, the "something else" does not fall automatically under the GPLv3, even if you distribute them together.
The runtime library must be accompanied by the preferred form for modification (source code), the "something else" can even be distributed as a binary.
Paolo
