Add rerun-if-env-changed=RUST_CHECK to librustc_llvm

Author: varkor

src/librustc_llvm/build.rs

@@ -30,6 +30,7 @@ fn detect_llvm_link() -> (&'static str, &'static str) {
 fn main() {
     if env::var_os("RUST_CHECK").is_some() {
         // If we're just running `check`, there's no need for LLVM to be built.
+        println!("cargo:rerun-if-env-changed=RUST_CHECK");
         return;
     }