title: Trouble with OPAM: External solver failure
body: <p>Trying to develop with OCaml on Arch Linux 4.12.10, I keep running into this error message:</p>
  
  <pre><code>### stderr ###
  # error: executing gringo failed (No such file or directory)
  # error: gringo returned with non-zero exit status
  
  [ERROR] External solver failure, please fix your installation and check /home/me/.opam/config and variable $OPAMEXTERNALSOLVER.
          You may also retry with option --use-internal-solver
  </code></pre>
  
  <p>This happens for example with these commands:</p>
  
  <ul>
  <li><code>opam init</code></li>
  <li><code>opam install ocamlfind</code></li>
  <li><code>opam switch 4.02.3+buckle-master</code></li>
  </ul>
  
  <p>Indeed, as the error message says, appending <code>--use-internal-solver</code> makes the commands run successfully.</p>
  
  <p>But is there a proper fix for this?</p>
  
  <p>My OPAM version is 1.2.2.</p>