Refactor mkFlag / shouldUsePkg into the nixpkgs libraries

pkgs/misc/jackaudio/default.nix

@@ -9,10 +9,9 @@
 , prefix ? ""
 }:
 
+with stdenv;
 with stdenv.lib;
 let
-  shouldUsePkg = pkg: if pkg != null && stdenv.lib.any (x: x == stdenv.system) pkg.meta.platforms then pkg else null;
-
   libOnly = prefix == "lib";
 
   optDbus = shouldUsePkg dbus;

pkgs/misc/jackaudio/jack1.nix

@@ -4,9 +4,8 @@
 , alsaLib ? null, db ? null, libuuid ? null, libffado ? null, celt ? null
 }:
 
+with stdenv;
 let
-  shouldUsePkg = pkg: if pkg != null && stdenv.lib.any (x: x == stdenv.system) pkg.meta.platforms then pkg else null;
-
   optAlsaLib = shouldUsePkg alsaLib;
   optDb = shouldUsePkg db;
   optLibuuid = shouldUsePkg libuuid;