Auto merge of #2138 - JohnTitor:rerun-if-changed, r=Amanieu

Specify `cargo:rerun-if-changed=build.rs` to avoid re-building

Fixes #1978
r? `@Amanieu`

build.rs

@@ -3,6 +3,9 @@ use std::process::Command;
 use std::str;
 
 fn main() {
+    // Avoid unnecessary re-building.
+    println!("cargo:rerun-if-changed=build.rs");
+
     let (rustc_minor_ver, is_nightly) =
         rustc_minor_nightly().expect("Failed to get rustc version");
     let rustc_dep_of_std = env::var("CARGO_FEATURE_RUSTC_DEP_OF_STD").is_ok();