trurl and C89

[jacobmealey]

Should trurl strive to be C89 compatible? Is that something we would want before a 1.0 release?

@jay layed out some of the work in the PR: (#268)

[bagder]

I'm personally quite used to writing C89 code from curl/libcurl, but I am not convinced it is important for trurl. I don't think this tool is likely to get built on platforms without support for C99.

If we want C89, then we should probably make sure a CI job makes sure to use options to complain if we fail to live up to it.