smpeg: unbreak on darwin

pkgs/development/libraries/smpeg/default.nix

···
 
   nativeBuildInputs = [ autoconf automake libtool m4 pkg-config makeWrapper ];
 
-  buildInputs = [ SDL gtk2 libGLU libGL ];
+  buildInputs = [ SDL ] ++ lib.optionals (!stdenv.isDarwin) [ gtk2 libGLU libGL ];
 
   outputs = [ "out" "dev" ];