It's way too hard to succeed `opam install ocsigen-start` on a fresh Debian 9

[pwbs]

We have to provide hints on how to install ocsigen-start dependencies (e.g., npm), including indirect dependencies (i.e., dependencies of packages that OS depends on and that opam won't install by itself).

[ghuysmans]

I think I've helped with 56ded05 but there are maybe a few missing depext I already had installed on my system...