Parse Jamfile to retrieve build-dir option

This feature was requested on Boost.Build list (http://lists.boost.org/boost-build/2013/12/27216.php)

> I'm now stuck on the --build-dir options as I'm setting the build-dir via the  "build-dir" option of the the project.
> Can this be parsed and the --build-dir option disabled? Clearing the path does not work right now too. 

It's a good idea to add.
