Remove unnecessary rerun-if-env-changed from build.rs

librustc_session already has them, and the crates depend on it, so
they will be rebuilt anyway

Author: luca-barbieri

src/librustc_ast/build.rs

@@ -1,6 +1,4 @@
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
-    println!("cargo:rerun-if-env-changed=CFG_VERSION");
-    println!("cargo:rerun-if-env-changed=CFG_PREFIX");
     println!("cargo:rerun-if-env-changed=CFG_LLVM_ROOT");
 }

src/librustc_codegen_llvm/build.rs

@@ -2,8 +2,6 @@ use std::env;
 
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
-    println!("cargo:rerun-if-env-changed=CFG_LIBDIR_RELATIVE");
-    println!("cargo:rerun-if-env-changed=CFG_COMPILER_HOST_TRIPLE");
     println!("cargo:rerun-if-env-changed=RUSTC_VERIFY_LLVM_IR");
 
     if env::var_os("RUSTC_VERIFY_LLVM_IR").is_some() {