Log Message:
Update to 2.4.0, from Jaap Boender in PR 46798:

This is a maintainer update of devel/ocaml-lwt to its newest version, 2.4.0. The
changelog (bugfixes and some enhancements, plus compatibility with ocaml 4.00,
though it should still compile with ocaml 3.12):

- Reimplement Lwt_stream
  much simpler and more efficient
  do not use Weak
  add bounded push streams
- Add Lwt.async
- Add Lwt_preemptive.run_in_main
- Implement Lwt_unix.get_credentials on MacOS X/OpenBSD
- Ensure that on_cancel functions are executed first
- Better implementation of Lwt.cancel with more tests
- Simplify the API for unix jobs
- Better handling of the master lock in libev stubs
- Windows fixes/updates:
  pass -lws2_32 instead of ws2_32.lib if building with mingw
  fix a bug causing Lwt_unix.read/write to block when a socket is not
        readable/writable
  port Lwt_process and Lwt_unix.system to windows
- Compatibility with ocaml 4.00:
  add O_SHARE_DELETE to Lwt_unix.open_flag
  add -package compiler-libs.toplevel for files using Toploop
- Do not use module Sys for signal handling to avoid ocaml code to be called in \ 
a C thread
- Fix Lwt_unix.wrap_syscall: try instead of Lwt.catch
- Fix a dead-lock between lwt_unix_send_notificati
- Fix #277: add a function to return the Ssl.socket of a Lwt_ssl.socket

There have also been a few changes to the package itself; it now uses
PLIST_VARS, and the patches are replaced by the SUBST mechanism.