title: opam fails trying to install lwt looking for pthreads
body: <p>On a Ubuntu Trusty VM, I've installed opam (version 1.2.0) from the ppa recommended by Real World Ocaml set up docs.</p>
  
  <p>When running </p>
  
  <pre><code>opam install lwt
  </code></pre>
  
  <p>the build fails, and examing the .out file shows:</p>
  
  <pre><code>testing for pthread: ........................... unavailable
  not checking for glib
  
  The following recquired C libraries are missing: pthread.
  </code></pre>
  
  <p>I have the Ubuntu package libpthread-stubs0-dev installed.  What do I need to install to let the install succeed?</p>