ocamlPackages_4_06.num: fix after #110571

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

···
 { stdenv, lib, fetchFromGitHub, fetchpatch, ocaml, findlib, withStatic ? false }:
 
-stdenv.mkDerivation rec {
+stdenv.mkDerivation (rec {
   version = "1.1";
   pname = "ocaml${ocaml.version}-num";
   src = fetchFromGitHub {
···
     inherit (ocaml.meta) platforms;
     inherit (src.meta) homepage;
   };
-}
+} // (if lib.versions.majorMinor ocaml.version == "4.06" then {
+    NIX_CFLAGS_COMPILE = "-fcommon";
+  } else {})
+)