squashfs-tools-ng: document the specific blocker for a darwin build

pkgs/tools/filesystems/squashfs-tools-ng/default.nix

@@ -19,6 +19,17 @@ stdenv.mkDerivation rec {
     license = licenses.gpl3Plus;
     maintainers = with maintainers; [ qyliss ];
     platforms = platforms.unix;
+
+    # TODO: Remove once nixpkgs uses newer SDKs that supports '*at' functions.
+    # Probably macOS SDK 10.13 or later. Check the current version in
+    # ../../../../os-specific/darwin/apple-sdk/default.nix
+    #
+    # From the build logs:
+    #
+    # > Undefined symbols for architecture x86_64:
+    # >   "_utimensat", referenced from:
+    # >       _set_attribs in rdsquashfs-restore_fstree.o
+    # > ld: symbol(s) not found for architecture x86_64
     broken = stdenv.isDarwin;
   };
 }