Per environment flags with js_of_ocaml

[pbiggar]

I want to use different js_of_ocaml flags in my project, depending on whether we are in dev or release profile.

Dune already adds --pretty and --source-maps-inline in dev, however, there are more jsoo options to play with, and I get dramatically better debugging with the recommended settings:

For Debugging: "–pretty –no-inline –debug-info" + eventually "–disable staticeval –disable share"
For Production: "–opt 3". It minimize the generated javascript by applying various optimizations until a fix-point is reached

I tried using an env block, but couldn't make it work any way I tried it. Is there an existing way to do this, and if not:

• can a way to do this be added?
• can we pick better defaults for the dev and production env?

[rgrinberg]

I have a PR that allows one to control these options, but it depends on #1044. Do you have an opinion on the proposal in that PR?

[Lupus]

I'm trying to get exception backtraces with jsoo by using flags --enable=with-js-error --enable=excwrap, but exception is being thrown by Lwt, and it seems that adding these flags to my binary does not change the way Lwt is being built, and thus I don't get the backtrace attached to the exception... Isn't there a way to set those globally? Or they have to be set within corresponding environment so that propagate to all jsoo invocations?

[hhugo]

Fixed by #5049