Since we are moving towards only depending on X11 for X-headers, superenv now doesn't automatically add X11 compilation. I was reluctant to do this, but it is the right thing to do now that X11 is not automatically installed by OS X or Xcode.
I didn't implement ENV.x11 because the order that the X headers are inserted is important. It must be done at initial setup to ensure that brewed versions of e.g. freetype and Cairo are used and not the ones installed by XQuartz.
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