MACOSX build fail with zlib-ng, upcoming fix upstream.

[iago-lito]

Our CI broke, possibly because of this issue in zlib-ng now resolved although the fix hasn't landed yet.

In the meantime, I though I could bump zlib-ng myself within my own clone of flate2-rs, just to test whether their develop branch would fix it, but I think I'm confused how it works.

Is this line in src/ffi/c.rs not the only one that needs bumping? What does the one in Cargo.toml mean then?
Does flate2-rs download and compile its own version of zlib-ng from sources? Does it rely on system libs instead? Do we have control over that?

Anyway, happy to help, but unsure where to start ^ ^"