title: How to install gem via Makefile if it doesn't exist
body: <p>I'm creating a <code>Makefile</code> for my project:</p>
  
  <pre><code>build:
      sudo gem install sass
  </code></pre>
  
  <p>Any time I build it's asking me for my superuser password. If I remove <code>sudo</code> it will not install at all, but throw an error instead, as I don't have permissions to install a <code>gem</code>.</p>
  
  <p>So I came up with an idea, that I want to check whether the <code>gem</code> already exists, and run installing command only when it doesn't.</p>
  
  <p>So the question is how to perform this check inside <code>Makefile</code>.</p>