Check that gzencode actually exists

I just spent a grueling hour figuring out why Boost wasn't caching on a *BSD server. It turns out BSD by default doesn't have the zlib library enabled (should have guessed grrrrr). My simple fix was adding this in settings.php:

  if (!function_exists('gzencode')){
    function gzencode($data){
      return $data;
    }
  }

To prevent issues with this it would probably be best to perform a check to see if gzencode actually exists and if not then Boost shouldn't call it. If anything, hopefully this will help people who might go through what I just went through. Welcome to the minimalistic nightmares of BSD.
A better fix would be a quick modification to