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

rust-lang/libc,rust-lang/libc,rust-lang/libc