autogen: try fixing on darwin

and cause no rebuild elsewhere.
In commit 2988780 I forgot that patchelf makes no sense there.

pkgs/development/tools/misc/autogen/default.nix

@@ -62,6 +62,7 @@ stdenv.mkDerivation rec {
       sed -e "s|$lib/lib|/no-such-autogen-lib-path|" -i $f
     done
 
+  '' + stdenv.lib.optionalString (!stdenv.hostPlatform.isDarwin) ''
     # remove /build/** from RPATHs
     for f in "$bin"/bin/*; do
       local nrp="$(patchelf --print-rpath "$f" | sed -E 's@(:|^)/build/[^:]*:@\1@g')"