Change:
<ins> { lib, stdenv, cmake, curl, curlpp, doctest, fetchFromGitHub, fetchurl, fmt, nlohmann_json, qt5, spdlog, substituteAll, trompeloeil, buildDayZLauncher ? false, }: stdenv.mkDerivation (finalAttrs: { pname = "arma3-unix-launcher"; version = "413"; src = fetchFromGitHub { owner = "muttleyxd"; repo = "arma3-unix-launcher"; rev = "2ea62d961522f1542d4c8e669ef5fe856916f9ec"; hash = "sha256-uym93mYmVj9UxT8RbwdRUyIPrQX7nZTNWUUVjxCQmVU="; }; patches = [ # prevent CMake from trying to get libraries on the internet (substituteAll { src = ./dont_fetch_dependencies.patch; argparse_src = fetchFromGitHub { owner = "p-ranav"; repo = "argparse"; rev = "45664c4e9f05ff287731a9ff8b724d0c89fb6e77"; sha256 = "sha256-qLD9zD6hbItDn6ZHHWBXrAWhySvqcs40xA5+C/5Fkhw="; }; curlpp_src = curlpp.src; doctest_src = doctest; fmt_src = fmt; nlohmann_json_src = nlohmann_json; pugixml_src = fetchFromGitHub { owner = "muttleyxd"; repo = "pugixml"; rev = "simple-build-for-a3ul"; sha256 = "sha256-FpREdz6DbhnLDGOuQY9rU17SSd6ngA4WfO0kGHqGJPM="; }; spdlog_src = spdlog; steamworkssdk_src = fetchurl { url = "https://github.com/julianxhokaxhiu/SteamworksSDKCI/releases/download/1.53/SteamworksSDK-v1.53.0_x64.zip"; sha256 = "sha256-6PQGaPsaxBg/MHVWw2ynYW6LaNSrE9Rd9Q9ZLKFGPFA="; }; trompeloeil_src = trompeloeil; }) # game won't launch with steam integration anyways, disable it ./disable_steam_integration.patch ]; nativeBuildInputs = [ qt5.wrapQtAppsHook cmake ]; buildInputs = [ spdlog curlpp.src curl qt5.qtbase qt5.qtsvg ]; cmakeFlags = [ "-Wno-dev" ] ++ lib.optionals buildDayZLauncher [ "-DBUILD_DAYZ_LAUNCHER=ON" ]; meta = { homepage = "https://github.com/muttleyxd/arma3-unix-launcher/"; description = "Clean, intuitive Arma 3 + DayZ SA Launcher"; license = with lib.licenses; [ # Launcher mit # Steamworks SDK unfree ]; maintainers = with lib.maintainers; [ DrymarchonShaun ]; mainProgram = "arma3-unix-launcher"; platforms = with lib.platforms; linux ++ darwin; }; }) </ins>

Discussion:
Project description A clean, intuitive Arma 3 + DayZ SA Launcher for GNU/Linux and MacOS. Metadata homepage URL: source URL: license: mit (does require the steamworks SDK which has its own license.) platforms: linux, darwin Add a :+1: [reaction] to [issues you find important]. [reaction]: [issues you find important]:
I was trying to package this myself, but the way cmake is set up has it trying to download dependencies with git, and modifying it to build properly with nix is over my head.
With a lot of trial and error, I got this functioning . No idea if my code is up to the standards for proper integration into nixpkgs, I'm not 100% sure if everything I have in there is even required, might have missed some failed attempts at fixes that aren't actually doing anything. Also haven't tested the version of the launcher for DayZ.
Looks great to me! Run it through to pretty it up and I think it'd fit in just fine in nixpkgs. I am getting a build error with the DayZ launcher, though: This might inspire me to go back to arma 3. Longtime OFP/arma fan!
What would I put for the license if the launcher itself is using MIT but the steam works sdk has its own custom license (not sure if it is considered unfree, either)
You can just include multiple licenses, though I'm not entirely sure what the Steamworks SDK license is, or if the license is in nixpkgs in the first place. I'd just mark it unfree if you don't know.