message: [fetch] Don't attempt to download on unknown OSes
diff: @@ -47,10 +47,13 @@ namespace vcpkg::Commands::Fetch
  static constexpr StringLiteral OS_STRING = "windows";
  #elif defined(__APPLE__)
  static constexpr StringLiteral OS_STRING = "osx";
  -#else // assume linux
  +#elif defined(__linux__)
  static constexpr StringLiteral OS_STRING = "linux";
  +#else
  + return ToolData{};
  #endif
  +#if defined(_WIN32) || defined(__APPLE__) || defined(__linux__)
  static const std::string XML_VERSION = "2";
  static const fs::path XML_PATH = paths.scripts / "vcpkgTools.xml";
  @@ -137,6 +140,7 @@ namespace vcpkg::Commands::Fetch
  paths.downloads / archive_name.value_or(exe_relative_path),
  tool_dir_path,
  sha512};
  +#endif
  }
  static bool exists_and_has_equal_or_greater_version(const std::string& version_cmd,
  