Auto merge of rust-lang#2515 - RalfJung:build, r=RalfJung

dont rerun build script unnecessarily

build.rs

@@ -1,4 +1,6 @@
 fn main() {
+    // Don't rebuild miri when nothing changed.
+    println!("cargo:rerun-if-changed=build.rs");
     // Re-export the TARGET environment variable so it can
     // be accessed by miri.
     let target = std::env::var("TARGET").unwrap();