Getopt library on Windows

[joto]

Building/Testing of some of the examples is disabled on Windows because of the missing getopt library. We should figure something out here.

See also mapbox/protozero#84.

[springmeyer]

I don't have a strong feeling on this. I'm okay with this being disabled on windows. I'd also be okay with moving to boost::program_options as long as the build auto-disabled features if boost is not available (like we do for variant, since it feels ideal to have vtzero easy to build without boost deps). Refs mapbox/cpp#35