title: Call llvm-config --prefix and use it in a BUILD rule
body: <p>I have a following rule in <strong>WORKSPACE</strong>:</p>
  
  <pre><code>new_local_repository(
      name = "llvm",
      path = "/opt/local/libexec/llvm-4.0",
      build_file= "llvm.BUILD")
  </code></pre>
  
  <p>I would like to now use hardcoded path for llvm. <code>llvm-config --prefix</code> can give me the directory for llvm. What is the correct way to get this? Can I just use standard python commands (e.g. <code>subprocess</code> package)?</p>