Fix some warnings that are returned with the GCC version I'm now using.
Still lots left to fix. Nothing that may lead to buggy behavior, I think.
Fix a lot of warnings on the CPU speed selector sprite.
Fortunately, this has made me look into that sprite... and find out that it was wasting 30 KB of binary space for a simple black-and-white image.
So I replaced it with a 1-bit sprite, which fixed the warnings and brought the add-in size down to 151 KB. Win-win.