racket: fix build on darwin

pkgs/development/interpreters/racket/default.nix

···
   shared = if stdenv.isDarwin then "dylib" else "shared";
   configureFlags = [ "--enable-${shared}"  "--enable-lt=${libtool}/bin/libtool" ]
                    ++ lib.optionals disableDocs [ "--disable-docs" ]
-                   ++ lib.optionals stdenv.isDarwin [ "--enable-xonx" ];
+                   ++ lib.optionals stdenv.isDarwin [ "--disable-strip" "--enable-xonx" ];
 
   configureScript = "../configure";