Fix MSVC build

[nojb]

The current main does not build under MSVC:

ocamlc -output-complete-exe -w -24 -g -o .duneboot.exe -I boot unix.cma boot/libs.ml boot/duneboot.ml
.\.duneboot.exe
cd _boot && C:\cygwin64\home\nojebar\mlfi\mlfi-ins\bin\ocamlopt.opt.exe -o ../_boot\dune.exe -g -I +threads unix.cmxa threads.cmxa -no-alias-deps -w -49-6 readdir.c wait3_stubs.c xdg_stubs.c dune_flock.c spawn_stubs.c dune_stats_stubs.c fsevents_stubs.c inotify_stubs.c pthread_chdir_stubs.c -args mods_list -cclib -lshell32 -cclib -lole32 -cclib -luuid -alert -unstable
C:\cygwin64\home\nojebar\dune-poll\dune\_boot\src\dune_util\dune_flock.c(33) : error C2220: l'avertissement suivant est traité comme une erreur
C:\cygwin64\home\nojebar\dune-poll\dune\_boot\src\dune_util\dune_flock.c(49) : error C2220: l'avertissement suivant est traité comme une erreur
C:\cygwin64\home\nojebar\dune-poll\dune\_boot\src\dune_util\dune_flock.c(33) : warning C4716: 'dune_flock_lock' : doit retourner une valeur
C:\cygwin64\home\nojebar\dune-poll\dune\_boot\src\dune_util\dune_flock.c(49) : warning C4716: 'dune_flock_unlock' : doit retourner une valeur
make: *** [Makefile:49: _boot/dune.exe] Error 2

[nojb]

If I understand correctly this is included in 3.6.0 (#6360), so it will need a bugfix release. cc @emillon.

[rgrinberg]

Can we test MSVC in CI as well?

[nojb]

Can we test MSVC in CI as well?

Not with any existing prepackaged workflow, as far as I know. The closest we have (again, as far as I know) is @jonahbeckford's https://github.com/diskuv/dkml-workflows (which is currently on 4.12 and is somewhat slow due to various technical reasons).

cc @jonahbeckford @dra27