message: Prepare for Rust 1.39
  This change is necessary to ensure that the files we create with bindgen
  still compile when we get to 1.39.
diff: @@ -231,7 +231,9 @@ fn build_bindings(base: &str, bindings: &Bindings, flags: &[String], gecko: bool
  println!("cargo:rerun-if-changed={}", header);
  - let mut builder = Builder::default().header(header).generate_comments(false);
  + let mut builder = Builder::default().header(header);
  + builder = builder.generate_comments(false);
  + builder = builder.derive_debug(false); // https://github.com/rust-lang/rust-bindgen/issues/372
  builder = builder.clang_arg("-v");
  