It would be interesting exercise to compile cells into object files as they are entered and load them dynamically into a persistent process, doing needed linking etc on the fly in-memory. You'd also need to generate a header file from the current in-memory symbol table and implicitly include that in compilation.

That might actually work relatively well for simple cases. But I imagine there would be a ton of problematic edge cases, considering that with C you can end up with all sorts of weird stuff.