i3: add configFile to enable cutom configuration locations

i3 loads its configuration from `~/.config/i3`, but in nix-based systems
you might want to build the config in `~/.nix-profile` using a nix
derivation, so `i3` needs to know where to look for the configuration
file.

pkgs/applications/window-managers/i3/default.nix

@@ -1,12 +1,14 @@
 { fetchurl, stdenv, which, pkgconfig, makeWrapper, libxcb, xcbutilkeysyms
 , xcbutil, xcbutilwm, xcbutilxrm, libstartup_notification, libX11, pcre, libev
 , yajl, xcb-util-cursor, coreutils, perl, pango, perlPackages, libxkbcommon
-, xorgserver, xvfb_run }:
+, xorgserver, xvfb_run, symlinkJoin, configFile ? null }:
 
-stdenv.mkDerivation rec {
-  name = "i3-${version}";
+let
   version = "4.13";
 
+  i3 = stdenv.mkDerivation rec {
+    name = "i3-${version}";
+
     src = fetchurl {
       url = "http://i3wm.org/downloads/${name}.tar.bz2";
       sha256 = "12ngz32swh9n85xy0cz1lq16aqi9ys5hq19v589q9a97wn1k3hcl";
@@ -69,4 +71,15 @@ stdenv.mkDerivation rec {
       '';
     };
 
+  };
+in if configFile == null then i3 else symlinkJoin {
+  name = "i3-with-config-${version}";
+  paths = [ i3 ];
+
+  buildInputs = [ makeWrapper ];
+
+  postBuild = ''
+      wrapProgram $out/bin/i3 \
+      --add-flags "-c ${configFile}"
+  '';
 }

pkgs/top-level/all-packages.nix

@@ -14483,6 +14483,8 @@ with pkgs;
 
   i3 = callPackage ../applications/window-managers/i3 {
     xcb-util-cursor = if stdenv.isDarwin then xcb-util-cursor-HEAD else xcb-util-cursor;
+
+    configFile = config.i3.configFile or null;
   };
 
   i3-gaps = callPackage ../applications/window-managers/i3/gaps.nix { };