title: How to specify an environment variable using the rustc-env flag?
body: <p>I want to set <code>rustc-env=VAR=VALUE</code> so that I could access it using <code>env::var("VAR")</code> in my code. However, I'm not clear on where to specify it. Can I set the environment variable <code>VAR</code> in the Makefile? </p>