Auto merge of #930 - RalfJung:build, r=RalfJung

test that build scripts do not run in Miri

@elichai reported something that sounded a lot like build script running in Miri. But as this test shows, build scripts are not run by Miri, they are run normally.

@elichai are you sure the [env var usage you were referring to](https://github.com/rust-lang/miri/issues/641#issuecomment-524989482) was only in a build script? Those shouldn't be affected by Miri flags at all. Is your code available somewhere so that I can try to reproduce?

Cargo.toml

@@ -5,7 +5,6 @@ license = "MIT/Apache-2.0"
 name = "miri"
 repository = "https://github.com/rust-lang/miri"
 version = "0.1.0"
-build = "build.rs"
 default-run = "miri"
 edition = "2018"
 

test-cargo-miri/build.rs

@@ -0,0 +1,15 @@
+#![feature(asm)]
+
+fn not_in_miri() -> i32 {
+    // Inline assembly definitely does not work in Miri.
+    let dummy = 42;
+    unsafe {
+        asm!("" : : "r"(&dummy));
+    }
+    return dummy;
+}
+
+fn main() {
+    not_in_miri();
+    println!("cargo:rerun-if-changed=build.rs");
+}