title: Homebrew shows "You have unlinked kegs in your Cellar" for cabal-install
body: <pre><code>$ brew doctor
  Please note that these warnings are just used to help the Homebrew maintainers
  with debugging if you file an issue. If everything you use Homebrew for is
  working fine: please don't worry and just ignore them. Thanks!
  
  Warning: You have unlinked kegs in your Cellar
  Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
  those kegs to fail to run properly once built. Run `brew link` on these:
  
      cabal-install
  </code></pre>
  
  <p>Should I really run <code>brew link</code>?   cabal-install is a separate system that install Haskell softwares and it can be updated in that system.</p>