What is the Arduino compiler's version of `__unused` for function parameters?
Well, I typed all that question up for posting, then tried one last thing. It worked!

void Fn(__attribute__((unused)) int param) {
} // Fn(param)


So, rather than let a good question go to waste: here's the answer!

Now all we need to do is get the maintainers of all the libraries to put these in... and fix the other warnings...