Global nav with dynamic links across multiple sites
At the moment, that's not possible with Craft itself. That might be possible in Craft 3, as cloning sites will be an option -- though all of your sites would also have to be in Craft. Not sure if sharing information between sites will exist in C3 or not though.

You COULD use Craft to create the navigation fairly easily. Then you could create a blank template (not extending your base layout) that only contains the navigation, and then loading that via AJAX on the other sites into a nav container div.