Move to C89

Currently, we are using ISO/IEC 9899:1999 (a.k.a. C99). C99 was chosen because it is hilarious to use an old spec of C to write an app that serves a HTML 5 webpage.

However, the oldest official standard for the C language is ANSI X3.159-1989 (a.k.a. C89).

Going back that far does bring back some dangerous language features (e.g. implicit function declarations), but as far as I can tell -Weverything complains if you use them. The main thing is only C-style comments are allowed and declaring variables is only allowed at the start of functions (even for for loops).

But there is only 1 instance of a declaration that needs moved, and the comments are easy to change, so the real-world impact of going back to C89 is minimal.

I am inclined to say the work is worth the amusement. Plus I would do the work. OK, I already did the work and have a branch ready to push. 😅

@qwindelzorf @Z3liff thoughts or concerns?