SDL2: do not patchelf libSDL2.so twice

There are two symlinks pointing to the same library, libSDL2.so and
libSDL2-2.0.so.0.  patchelf should run on non-symlinks instead.

pkgs/development/libraries/SDL2/default.nix

@@ -80,7 +80,7 @@ stdenv.mkDerivation rec {
   # from above in this way. This is pretty weird.
   postFixup = optionalString (stdenv.hostPlatform.extensions.sharedLibrary == ".so") ''
     for lib in $out/lib/*.so* ; do
-      if [[ -L "$lib" ]]; then
+      if ! [[ -L "$lib" ]]; then
         patchelf --set-rpath "$(patchelf --print-rpath $lib):${lib.makeLibraryPath propagatedBuildInputs}" "$lib"
       fi
     done