Tell libev to use kqueue by default on macOS

[aantron]

On macOS, libev seems to default, too often, to select(2). Given that a major reason for using libev in the first place is to avoid select(2), we should have Lwt ask libev for its kqueue backend explicitly.

In other words, on macOS, new Lwt_engine.libev () should default to new Lwt_engine.(libev ~backend:Ev_backend.kqueue ()). See class Lwt_engine.libev.

cc @yallop, @anmonteiro as people who have been bitten by this.

[aantron]

See also #269, in which @yallop added support for choosing the backend.

[aantron]

libev should probably also be chosen by default on mac:

lwt/src/unix/config/discover.ml

Lines 860 to 868 in ee466e2

	match Configurator.ocaml_config_var_exn context "system" with
	| "linux"
	| "linux_elf"
	| "linux_aout"
	| "linux_eabi"
	| "linux_eabihf" ->
	true
	| _ ->
	false

[aantron]

This also seems to be a duplicate of #87.

[Marchhill]

I am currently having problems with this. When I run:
Lwt_engine.set (new Lwt_engine.libev ~backend:Lwt_engine.Ev_backend.kqueue ());
I get a segmentation fault. Are there any workarounds for this?
Platform: macOS 12.5

[raphael-proust]

@Marchhill what Lwt version are you using? Can you reproduce on the HEAD of the master branch?
What ocaml version are you using?

I don't have a mac so I can't reproduce, but having the added info could help someone else check.