[AIX] Use zlibNX if available.

[amy-kwan]

This change prevents libz-sys from building its own zlib by using zlibNX if available, which is an version of the zlib library that supports hardware accelerated data compression and decompression.

[Copilot]

Pull request overview

Updates libz-sys’s build script to prefer the AIX zlibNX installation when present, avoiding building a bundled zlib in that case.

Changes:

• Detect AIX targets and probe /usr/opt/zlibNX for libz.a and zlib.h.
• When found, configure Cargo to link against -lz from zlibNX and expose the include path, then exit early.

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[Copilot]

Local variable names zlibNX_lib/zlibNX_include don’t follow Rust’s usual snake_case naming used throughout this file (e.g., want_static, host_and_target_contain). Consider renaming to snake_case (e.g., zlibnx_lib, zlibnx_include or zlibnx_lib_dir, zlibnx_include_dir) for consistency.

Suggested change

	let zlibNX_lib = "/usr/opt/zlibNX/lib";
	let zlibNX_include = "/usr/opt/zlibNX/include";
	if std::path::Path::new(zlibNX_lib).join("libz.a").exists() &&
	std::path::Path::new(zlibNX_include).join("zlib.h").exists() {
	println!("cargo:rustc-link-search=native={}", zlibNX_lib);
	println!("cargo:rustc-link-lib=z");
	println!("cargo:include={}", zlibNX_include);
	let zlibnx_lib = "/usr/opt/zlibNX/lib";
	let zlibnx_include = "/usr/opt/zlibNX/include";
	if std::path::Path::new(zlibnx_lib).join("libz.a").exists() &&
	std::path::Path::new(zlibnx_include).join("zlib.h").exists() {
	println!("cargo:rustc-link-search=native={}", zlibnx_lib);
	println!("cargo:rustc-link-lib=z");
	println!("cargo:include={}", zlibnx_include);

[Byron]

Thanks a lot. Here is a review by Codex, based on my hunch, which ended up P1. Please do also take a look at P2, which may or may not make sense.

Thanks

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• P1: Honor explicit static mode before preferring zlibNX
  In build.rs, the new AIX fast-path returns as soon as /usr/opt/zlibNX exists, before should_link_static() is evaluated. That changes existing behavior: callers using the static feature or LIBZ_SYS_STATIC=1 can no longer force the bundled static build when zlibNX is present. This regresses a supported build mode.

• P2: Validate zlibNX is actually linkable before skipping fallback
  In build.rs, the AIX special case short-circuits based only on file existence. The current non-Windows system path only avoids the vendored build after zlib_installed() successfully compiles and links src/smoke.c. If /usr/opt/zlibNX exists but is unusable for the active compiler configuration, this change will fail later at link time instead of falling back to the bundled zlib.

[amy-kwan]

Thanks a lot. Here is a review by Codex, based on my hunch, which ended up P1. Please do also take a look at P2, which may or may not make sense.

Thanks

• P1: Honor explicit static mode before preferring zlibNX
  In build.rs, the new AIX fast-path returns as soon as /usr/opt/zlibNX exists, before should_link_static() is evaluated. That changes existing behavior: callers using the static feature or LIBZ_SYS_STATIC=1 can no longer force the bundled static build when zlibNX is present. This regresses a supported build mode.
• P2: Validate zlibNX is actually linkable before skipping fallback
  In build.rs, the AIX special case short-circuits based only on file existence. The current non-Windows system path only avoids the vendored build after zlib_installed() successfully compiles and links src/smoke.c. If /usr/opt/zlibNX exists but is unusable for the active compiler configuration, this change will fail later at link time instead of falling back to the bundled zlib.

Ah, those are good points. Thank you for the review, @Byron! Appreciate it!
I've attempted to address these comments in the newest commit.