ocamlPackages.nonstd,facile: run `dune upgrade` for newer ocaml versions

This allows us to build the packages on OCaml >=4.12

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

pkgs/development/ocaml-modules/facile/default.nix

@@ -1,11 +1,9 @@
-{ lib, fetchurl, buildDunePackage }:
+{ lib, fetchurl, buildDunePackage, ocaml }:
 
 buildDunePackage rec {
   pname = "facile";
   version = "1.1.4";
 
-  useDune2 = false;
-
   src = fetchurl {
     url = "https://github.com/Emmanuel-PLF/facile/releases/download/${version}/facile-${version}.tbz";
     sha256 = "0jqrwmn6fr2vj2rrbllwxq4cmxykv7zh0y4vnngx29f5084a04jp";
@@ -13,6 +11,9 @@ buildDunePackage rec {
 
   doCheck = true;
 
+  useDune2 = lib.versionAtLeast ocaml.version "4.12";
+  postPatch = lib.optionalString useDune2 "dune upgrade";
+
   meta = {
     homepage = "http://opti.recherche.enac.fr/facile/";
     license = lib.licenses.lgpl21Plus;

pkgs/development/ocaml-modules/nonstd/default.nix

@@ -1,11 +1,9 @@
-{ lib, fetchzip, buildDunePackage }:
+{ lib, fetchzip, buildDunePackage, ocaml }:
 
 buildDunePackage rec {
   pname = "nonstd";
   version = "0.0.3";
 
-  useDune2 = false;
-
   minimalOCamlVersion = "4.02";
 
   src = fetchzip {
@@ -13,6 +11,8 @@ buildDunePackage rec {
     sha256 = "0ccjwcriwm8fv29ij1cnbc9win054kb6pfga3ygzdbjpjb778j46";
   };
 
+  useDune2 = lib.versionAtLeast ocaml.version "4.12";
+  postPatch = lib.optionalString useDune2 "dune upgrade";
   doCheck = true;
 
   meta = with lib; {