title: How to use different ghc builds with stack?
body: <p>I would like to have different, non-ABI-compatible, builds of the same version of GHC, and use them in different projects.</p>
  
  <p>(In my case, the difference between the builds is integer-simple vs gmp.)</p>
  
  <p>The comments in <a href="https://github.com/commercialhaskell/stack/issues/399" rel="noreferrer">this issue</a> show how to add a custom flavor of ghc to <code>stack-setup-2.yaml</code>. But how do I specify which ghc build to use in each particular case or project?</p>
  
  <p>I.e. I am looking for one (better yet, both) of:</p>
  
  <ol>
  <li>The ability to specify in <code>stack.yaml</code> whether to use the integer-simple or gmp build.</li>
  <li>The ability to specify, at <code>stack build</code> time, which build to produce.</li>
  </ol>