How to drupal_add_js an external and defer script
Just add a value in $options

$options = array(
  'type' =&gt; 'external',
  'defer' =&gt; TRUE,
);
drupal_add_js('http://example.com/whatever.js', $options);


From the documentation:


  Parameters
  
  $options: (optional) A string defining the type of JavaScript that is being added in the $data parameter ('file'/'setting'/'inline'/'external'), or an associative array. JavaScript settings should always pass the string 'setting' only. Other types can have the following elements in the array:
  
  
  type: The type of JavaScript that is to be added to the page. Allowed values are 'file', 'inline', 'external' or 'setting'. Defaults to 'file'.
  ...
  defer: If set to TRUE, the defer attribute is set on the  tag. Defaults to FALSE.