diff --git a/boring-sys/build.rs b/boring-sys/build.rs
index 55dae86d..07d5bd35 100644
--- a/boring-sys/build.rs
+++ b/boring-sys/build.rs
@@ -355,7 +355,8 @@ fn main() {
     println!("cargo:rustc-link-lib=static=ssl");
 
     // MacOS: Allow cdylib to link with undefined symbols
-    if cfg!(target_os = "macos") {
+    let target_os = std::env::var("CARGO_CFG_TARGET_OS").unwrap();
+    if target_os == "macos" {
         println!("cargo:rustc-cdylib-link-arg=-Wl,-undefined,dynamic_lookup");
     }