Prevent build.rs rerunning unnecessarily on all source changes

Author: dtolnay

build.rs

@@ -3,6 +3,8 @@ use std::process::Command;
 use std::str;
 
 fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
     let compiler = match rustc_minor_version() {
         Some(compiler) => compiler,
         None => return,