Where to put third party PHP library?

Question

I'm developing a couple of open-source plugins and a theme (all part of a "suite") that all use the same third party PHP library. I'm wondering what is the best way to include it in Wordpress. Here are some thoughts:

• put it in one of the plugins and require that plugin to be installed and activated
• create a special "core" plugin that does nothing but including it
• put it directly in wp-content

Any thoughts on that matter?