code-browser: ensure only one backend is enabled

+2
+2
pkgs/applications/editors/code-browser/default.nix
··· 8 8 , withGtk3 ? false, gtk3 9 9 , mkDerivation ? stdenv.mkDerivation 10 10 }: 11 + let onlyOneEnabled = xs: 1 == builtins.length (builtins.filter lib.id xs); 12 + in assert onlyOneEnabled [ withQt withGtk2 withGtk3 ]; 11 13 mkDerivation rec { 12 14 pname = "code-browser"; 13 15 version = "8.0";