-Werror

[Dan Eble]

Han-Wen proposed building with -Werror in a merge request.  What do you think?

I like -Werror, but I've only ever used it where there were very few (or one) 
supported build environments, all of which were covered in CI.  A dimension of 
the CI coverage was optimization level, which can change what the compiler 
discovers.

We don't have that here, so we might regret turning warnings into errors all at 
once.  How about we turn particular warnings into errors, starting with 
narrowing conversions and printf formatting, and wait to see if anyone reports 
problems?  (Am I being too conservative?)
— 
Dan