title: boost::filesystem::unique_path returns empty string on android
body: <p>On android, this:</p>
  
  <pre><code>std::string path = boost::filesystem::unique_path().string();
  </code></pre>
  
  <p>returns the empty string. No exception or other error condition that I can detect. Specifically this is a Nexus 4 running Android 4.4.2. Any idea what's up here?</p>