Skip rerunning build script on library code changes

Author: dtolnay

impl/build.rs

@@ -1,6 +1,7 @@
 fn main() {
     // Warning: build.rs is not published to crates.io.
 
+    println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-cfg=check_cfg");
     println!("cargo:rustc-check-cfg=cfg(check_cfg)");
     println!("cargo:rustc-check-cfg=cfg(exhaustive)");