Revert "'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."

This reverts commit ceccb2775510cf81f93605d0fac930b4c6d336d0.
Revert "configure ocamlnet without lablgtk because it doesn't work: it wrongly complains about the version of lablgtk being out of date"

This reverts commit 6140bbd1b9eb4a650feb06c4cbde64c572a33e8b.
Revert "add index.tar.gz and urls.txt (generated by opam-amdin) to .gitignore"

This reverts commit 527dfe06dcf4e0131005080edf5cd45a95bede7f.