oqgraph will be automatically built when libboost is available. In order to
follow separation from operating system avoid to building this not needed engine
and thus do not depend on operating system's library.

I updated the dropbear url because it wasn't uploaded to network cache.
I kept an old version because :
 * our patches don't work anymore with the latest version.
 * the latest version (at the date of this commit, doesn't have an
   interesting changelog).
 * by laziness.