Prevent build.rs rerunning unnecessarily on all source changes

serde_derive_internals/build.rs

@@ -1,6 +1,9 @@
 use std::path::Path;
 
 fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=src/mod.rs");
+
     // Sometimes on Windows the git checkout does not correctly wire up the
     // symlink from serde_derive_internals/src to serde_derive/src/internals.
     // When this happens we'll just build based on relative paths within the git