fn generate_user_agent_vars(output_path: &Path) {
    let rust_version = rustc_version::version().expect("Could not retrieve rustc version");
    let mut f = File::create(&output_path.join("user_agent_vars.rs"))
        .expect("Could not create user agent file");
    f.write_all(format!("static RUST_VERSION: &str = \"{}\";", rust_version).as_bytes())
        .expect("Unable to write user agent");
}
Parses and generates variables used to construct a User-Agent.

This is used to create a User-Agent header string resembling
`rusoto/x.y.z rust/x.y.z <os>`.
rusoto/core/build.rs