title: Understand homebrew and keg-only dependencies
body: <p>I've recently started using homebrew, and I'm a bit confused as to what happens when I brew something onto my system, but its brewed dependencies are keg-only, meaning that they are linked under <code>/usr/local</code>.</p>
  
  <p>For example, I'm install vips (an image processing library). One of its many dependencies is pixman. Pixman is installed as keg-only because apparently Mac OSX (Snow Leopard in this case) already ships with it (or a version of it).</p>
  
  <p>Does this mean that vips will use the system version of pixman? If so, aren't there potential versioning issues related to using the system version rather than the dependency version (assuming they are different)?</p>