I want to include the URL of jQuery UI http://code.jquery.com/ui/1.9.0/jquery-ui.js in my JSF page. I saw many questions which said
Just use plain HTML <script> element the usual way.
<script src="http://code.jquery.com/ui/1.9.0/jquery-ui.js"></script>
The <h:outputScript> can only refer local scripts. You'll only miss its dynamic versioning, modularity and relocation advantages, but that should technically not harm for a static and external script.
It's however possible to use a custom ResourceHandler to change the URL of a <h:outputScript> to be an external URL for pure CDN purposes. OmniFaces CDNResourceHandler is such an example.
Unrelated to the concrete problem, PrimeFaces components are built around jQuery/UI. Are you absolutely positive that you need a separate instance of jQuery UI library?