Prevent build.rs rerunning unnecessarily on all source changes

serde_test/build.rs

@@ -6,6 +6,8 @@ use std::str::{self, FromStr};
 // opening a GitHub issue if your build environment requires some way to enable
 // these cfgs other than by executing our build script.
 fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
     let minor = match rustc_minor_version() {
         Some(minor) => minor,
         None => return,