Fixing compilation and byte-compilation warnings before 25.1

[John Wiegley]

I'm doing an Emacs build now to test a patch, and it occurs to me how many
warnings and byte-compilation warnings I'm seeing. I'd like us to resolve as
many of these warnings as we can before shipping. It makes the build output
look a bit... unkempt.

Anyone interested in becoming a champion for this cause?

John