fix: Support opt-out via LIBZ_SYS_STATIC=0

This change refactors the `want_static` logic to additionally support `LIBZ_SYS_STATIC=0` as an explicit opt-out to the `static` feature when `libz-sys` is a transitive dependency and the build environment prefers to dynamic link `libz` instead.

Author: polarathene

build.rs

@@ -30,12 +30,11 @@ fn main() {
         return;
     }
 
+    let want_static = should_link_static();
     // Don't run pkg-config if we're linking statically (we'll build below) and
     // also don't run pkg-config on FreeBSD/DragonFly. That'll end up printing
     // `-L /usr/lib` which wreaks havoc with linking to an OpenSSL in /usr/local/lib
     // (Ports, etc.)
-    let want_static =
-        cfg!(feature = "static") || env::var("LIBZ_SYS_STATIC").unwrap_or(String::new()) == "1";
     if !want_static &&
        !target.contains("msvc") && // pkg-config just never works here
        !(host_and_target_contain("freebsd") ||
@@ -238,3 +237,22 @@ fn zlib_installed(cfg: &mut cc::Build) -> bool {
 
     false
 }
+
+// The environment variable `LIBZ_SYS_STATIC` is first checked for a value of `0` (false) or `1` (true),
+// before considering the `static` feature when no explicit ENV value was detected.
+// When `libz-sys` is a transitive dependency from a crate that forces static linking via the `static` feature,
+// this enables the build environment to revert that preference via `LIBZ_SYS_STATIC=0`.
+// The default is otherwise `false`.
+fn should_link_static() -> bool {
+  let has_static_env: Option<&'static str> = option_env!("LIBZ_SYS_STATIC");
+  let has_static_cfg = cfg!(feature = "static");
+
+  has_static_env
+    .and_then(|s: &str| s.parse::<u8>().ok())
+    .and_then(|b| match b {
+      0 => Some(false),
+      1 => Some(true),
+      _ => None,
+    })
+    .unwrap_or(has_static_cfg)
+}