title: How do I 'pass down' feature flags to subdependencies in Cargo?
body: <p>I'm writing a library in Cargo. If this library depends on another library like <a href="http://crates.io/crates/libc" rel="noreferrer"><code>libc</code></a>, which exposes a feature (in this case, <code>use_std</code>), how do I make a feature I expose enable or disable that feature in my dependency?</p>
  
  <p>Looking at the <a href="http://doc.crates.io/manifest.html#the-features-section" rel="noreferrer">cargo documentation</a>, it looks like there's no official way specified to do this.</p>