SCons: ability to define additional libraries in a flexible way
Build: make dependency on X-Lib explicit

The XV-Viewer widget in our GUI uses four direct calls
to the X-Lib. This was discovered by strict dependency checking,
as mandated by new Debian policy
Debian/Policy 3.9.x : enforce strict dependencies on dynamic modules

The recommendation is to use the link flag --no-undefined
and to fed *all* dependencies to the respective link step.

This changeset enables this strict linking of dependencies.
It turned out that our dependencies were already sane
(with the sole exception of a direct dependency to X-Lib
in the XV viewer widget)