Allow linking against system zlib-ng

[figsoda]

Currently it always(*) builds the vendored zlib-ng when zlib-ng feature is enabled

[happysalada]

I am interested in this feature too.
Some environment_variable like ZLIB_NG_DIR or something in the like would be amazing.
Happy to implement this if there it is allowed.

[chenrui333]

any updates on this?

[musicinmybrain]

I don’t have a nice upstreamable patch for conditionalizing this, but I can at least share how we unconditionally link the system library in Fedora’s rust-libz-ng-sys package. Working from the released libz-ng-sys crate:

1. Remove src/zlib-ng/ to prove it’s not used.
2. Remove cmake from the build-dependencies in Cargo.toml since we don’t need to build our own copy of zlib-ng.
3. Entirely replace build_zng.rs with the following trivial version:

fn main() {
    println!("cargo:rustc-link-lib=dylib=z-ng");
    println!("cargo:rustc-cfg=zng");
}

Hope that’s helpful for some people.

[musicinmybrain]

The above comment was written based on 1.1.15. For the just-released 1.1.16, the script replaces zng/cmake.rs instead of build_zng.rs.

Also, you may consider the trivial script in #83 (comment) to be contributed under the same MIT OR Apache-2.0 license as the crate.