It's good practise not to bundle foreign code. libbson-1.1.2 bundles YAJL library and uses it's internals not int the public YAJL API.

Attached patch introduces a configure-time check for system YAJL library. If it's available, it is used instead of the bundled one.

It would be good if libbson stopped using YAJL internals, or if you persuaded YAJL maintainers to publish API you need.