Add nix-support/private to unfree derivation outputs

overlay.nix

@@ -7,6 +7,32 @@ let
   callPackage = final.callPackage;
 
   bscPkgs = {
+
+    # override stdenv to add a sentinel to know if a derivation comes
+    # from unfree sources.
+    stdenv = prev.stdenv // {
+      mkDerivation =
+        args:
+        let
+          originalDrv = prev.stdenv.mkDerivation args;
+          checkLicense = l: if builtins.isAttrs l then !(l.free or true) else false;
+
+          licenses = if builtins.isList args.meta.license then args.meta.license else [ args.meta.license ];
+
+          hasUnfreeLicense =
+            if args ? meta && args.meta ? license then builtins.any checkLicense licenses else false;
+        in
+        if hasUnfreeLicense then
+          originalDrv.overrideAttrs (old: {
+            postInstall = (old.postInstall or "") + ''
+              mkdir -p $out/nix-support
+              touch $out/nix-support/private
+            '';
+          })
+        else
+          originalDrv;
+    };
+
     bench6 = callPackage ./pkgs/bench6/default.nix { };
     bigotes = callPackage ./pkgs/bigotes/default.nix { };
     clangOmpss2 = callPackage ./pkgs/llvm-ompss2/default.nix { };