http://www.ros.org/debbuild/jade.html?q=diff

Starting from MongoDB 2.6, C++ client libraries are not included anymore in Ubuntu. They are at https://github.com/mongodb/mongo-cxx-driver . This is SCons: we would need to bloom it ....

BUT, the libmongo-clients packages actually contains the C client library. So, we could also switch to it. Your call.