Patches for OCamlGraph.1.8.3 and Frama-C.20130601, and new package conf-gnomecanvas 

- First commit (and third, that just wrap a dependency line) adds a package conf-gnomecanvas that checks whether libgnomecanvas is present (on the model of conf-gtksourceview) for packages that require that in addition to gtk (lablgtk provides bindings for gnomecanvas if the library is present), and put appropriate dependencies on this package for OCamlgraph, Frama-C and lablgtk.  
- Second commit provides a patch for Frama-C.20130601 so that it compiles with 4.01beta1
- Fourth commit fixes the installation of OCamlGraph, who didn't export dgraph.cmi, the interface for its GUI part.