These are great news. Thank you!

Oh, you are right, it's not upstream. It's actually applied to Homebrew (https://brew.sh/) libtool formula which is where I originally got libtool.m4, see:

That would work, however we might hit this issue with a new macOS release. Would it make more sense to update to what the new libtool.m4 is doing? Linker flags are the same, it only changes the way they detect macOS versions: