weechat-xmpp: create proper runtime environment

The `weechat-xmpp` plugin requires `pydns` to properly resolve the
XMPP host. Furthermore it's much easier to use substituteAll rather than
messing around with substituteInPlace in a patched file.

pkgs/applications/networking/instant-messengers/weechat-xmpp/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchFromGitHub, xmpppy }:
+{ stdenv, fetchFromGitHub, xmpppy, pydns, substituteAll, buildEnv }:
 
 stdenv.mkDerivation {
   name = "weechat-jabber-2017-08-30";
@@ -15,15 +15,14 @@ stdenv.mkDerivation {
     cp jabber.py $out/share/jabber.py
   '';
 
-  buildInputs = [ xmpppy ];
-
-  postPatch = ''
-    substituteInPlace jabber.py \
-      --replace "__NIX_OUTPUT__" "${xmpppy}/lib/python2.7/site-packages"
-  '';
-
   patches = [
-    ./libpath.patch
+    (substituteAll {
+      src = ./libpath.patch;
+      env = "${buildEnv {
+        name = "weechat-xmpp-env";
+        paths = [ pydns xmpppy ];
+      }}/lib/python2.7/site-packages";
+    })
   ];
 
   meta = with stdenv.lib; {

pkgs/applications/networking/instant-messengers/weechat-xmpp/libpath.patch

@@ -8,7 +8,7 @@ index 27006a3..e53c2c0 100644
  
 +import sys
 +
-+sys.path.append('__NIX_OUTPUT__')
++sys.path.append('@env@')
 +
 +
  import_ok = True

pkgs/top-level/all-packages.nix

@@ -18982,7 +18982,9 @@ with pkgs;
     inherit (luaPackages) cjson;
   };
 
-  weechat-xmpp = callPackage ../applications/networking/instant-messengers/weechat-xmpp {};
+  weechat-xmpp = callPackage ../applications/networking/instant-messengers/weechat-xmpp {
+    inherit (pythonPackages) pydns;
+  };
 
   westonLite = weston.override {
     pango = null;