title: Podspec. Set s.dependency to download forked repo sources
body: <p>I have a repo that I download via cocoa pods. This repo includes .podspec file which include <code>s.dependency 'glm', '~&gt; 0.9.4.6'</code> line.</p>
  
  <p>as I understood it will download it from here: <a href="https://github.com/g-truc/glm" rel="nofollow">https://github.com/g-truc/glm</a></p>
  
  <p>I forked glm and now I want to use <code>s.dependency</code> that is connected to forked repo of glm. How can I specify it in podspec file to point that I need to download forked glm sources?</p>