Option to use zopfli from the system.

[Code7R]

Prep work for https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1135289

[Code7R]

@madler That said, would it be okay to cherry-pick this change and use it with stable version 2.8? I.e. I would like to have your blessing to release a package with this modification and linked to shared lib in Debian.

[madler]

I don't think you need my permission, but sure, so long as you test it well. It's been a while since I pulled changes from the zopfli repository, so I'm sure the Debian shared library is a more recent version. I don't know if there might have been incompatible changes.

[Code7R]

I prefer to have at least some communication about intended (non-trivial) changes. Some software developers are really sensitive on that topic.

So I will make a package release with that mod (and STDOUT help change, see another MR) ASAP. To Debian Unstable with the longest testing period. You can consider us a test ground for this MR.

Thanks, and have a nice day!