This may help you. After 5-seconds have passed, Javascript checks if jQuery is available, if not, then loads the library from local server.
1. With a timer
<script>
setTimeout(function() {
if(window.jQuery) return;
var n = document.getElementsByTagName("script")[0];
n.parentNode.insertBefore(document.createElement("script"), n).src = "assets/jQuery/jquery-1.7.2.min.js";
}, 5000);
</script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.0/jquery.min.js"></script>
2. This one doesn't have a timer, it loads a local version if the CDN version fails.
<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.js"></script>
<script type="text/javascript">window.jQuery || document.write("<script type='text/javascript' src='js/jquery-1.8.3.min.js'>\x3C/script>")</script>