Motivation for this change

Personally tested to work on macOS.

Cygwin port: https://github.com/fd00/yacp/tree/master/darkhttpd
OpenBSD port: http://ports.su/www/darkhttpd

From darkhttpd's site:

At some point worked on FreeBSD, Linux, OpenBSD, Solaris.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.