message: Add a comment indicating what generates this file.
diff: @@ -2,6 +2,8 @@ use cbindgen::{Builder, Language};
  use std::{env, fs, path::PathBuf};
  const PRE_HEADER: &'static str = r#"
  +// This file is generated by lib/c-api/build.rs.
  +
  // Define the `ARCH_X86_X64` constant.
  #if defined(MSVC) && defined(_M_AMD64)
  # define ARCH_X86_64
  