Decide on a C standard to target

[pngwen]

Discuss C versions and select the target:

Choice:

• ANSI (C89)
• C99
• C11
• C18

[de01dwc]

May be worth noting from Raylib's repository...

• Written in plain C code (C99) using PascalCase/camelCase notation

[josiahtripp]

I think C99 would be a good version to go with.
As Matt pointed out, it's the same version that raylib is written in, so maybe it'll help eliminate any possible incompatibilities.
While C11 & C18 may have a few minor features that C99 doesn't posses, I don't think we'd really need them, since most of the differences seem to pertain to aspects that are insignificant to xboing, such as some macros & threads.