title: Cocoapods and custom xcconfig
body: <p>I am trying to use Cocoapods with some custom configurations in an iOS project.<br>
  I have 3 (Dev, Stage, Prod) and each of them has some custom <code>GCC_PREPROCESSOR_DEFINITIONS</code>.
  I have seen around people suggesting to us <code>#include &lt;path-to-pods.xcconfig&gt;</code>, but this seems to the old way to do this.<br>
  I have seen Cocoapods <code>0.39</code> is automatically generating its config files based on my configurations and adding them to my targets automatically (and this is good).<br>
  This is also confirmed by <a href="https://christiantietze.de/posts/2015/10/cocoapods-custom-build-configuration/" rel="noreferrer">this article</a> who is talking about a "new way" to create Podfiles.
  The problem is these files don't contain my configurations.<br>
  I was trying to use <code>xcodeproj</code> and <code>link_with</code>, but without success.
  Does anyone know what is the correct way to deal with Cocoapods + custom xcconfig files?  </p>