configure ocamlnet without lablgtk because it doesn't work: it wrongly complains about the version of lablgtk being out of date
'opam install bson' works, but 'opam install js_of_ocaml bson' fails because 'deriving' is needed when 'js_of_ocaml' is present, but both are listed as optional dependencies. Solution: move 'deriving' from the list of optional to strong dependencies.