getopt VS cargs

Between -std=c99 and removing these headers, you're missing time definitions (struct timeval, gettimeofday) and option parsing definitions (struct option, getopt_long). Mingw-w64 provides all this for compatibility, but MSVC has none of these, so you'll need to write replacements. I've written embeddable, public domain implementations of getopt and something like getopt_long, in case that helps. These are how I deal with option parsing portability.

The option parser is crude and doesn't follow conventions. Particularly the lack of -- support (disables option parsing) would make it impossible to use safely in scripts. If you don't want to use the system or toolchain-provided getopt, here's a public domain, embeddable implementation: getopt.h

On unix-like systems there's a getopt function for parsing short options. Mingw-w64 has one as well, to cover Windows programs. If I care about portability, I just embed my own so it not only works everywhere, it behave the same everywhere, too.

https://github.com/skeeto/getopt/blob/master/getopt.h (short)

Not only the GNU, you have an example in this thread where someone suggested to use or roll something similar to https://github.com/skeeto/getopt/blob/master/getopt.h, which, surprise, depends on strchr, a function that solves a much trivial problem than POSIX getopt, to begin with.

getopt is definitely a solid option and a good choice for suggesting to beginners. I personally prefer cargs, and I never use getops anymore. You can check out cargs here: https://github.com/likle/cargs