core-unix 0.17.1 not working on Windows

[hannesm]

Dear everyone,

I don't exactly know how to mark this unavailable on windows (esp with #26089 in my mind). But I see these failures regularly, and it would be nice to mark them properly.

  #=== ERROR while compiling core_unix.v0.17.1 ==================================#
  # context     2.4.1 | win32/x86_64 | ocaml.5.3.0 | file://D:/a/opam-repository/opam-repository
  # path        D:\opamroot\default\.opam-switch\build\core_unix.v0.17.1
  # command     D:\opamroot\default\bin\dune.exe build -p core_unix -j 3
  # exit-code   1
  # env-file    D:\opamroot\log\core_unix-5712-ba63ce.env
  # output-file D:\opamroot\log\core_unix-5712-ba63ce.out
  ### output ###
  # [...]
  # syslog_stubs.c:2:10: fatal error: syslog.h: No such file or directory
  #     2 | #include <syslog.h>
  #       |          ^~~~~~~~~~
  # compilation terminated.
  # File "lock_file_blocking/src/dune", lines 9-13, characters 0-96:
  #  9 | (rule
  # 10 |  (targets config.h)
  # 11 |  (deps)
  # 12 |  (action
  # 13 |   (bash "cp %{lib:jst-config:config.h} %{targets}")))
  # (cd _build/default/lock_file_blocking/src && D:\opamroot\.cygwin\root\bin\bash.exe -e -u -o pipefail -c "cp D:\opamroot\default\lib\jst-config\config.h config.h")
  # cp: cannot stat 'D:opamrootdefaultlibjst-configconfig.h': No such file or directory

I suspect other versions of core_unix as well are not supported on Windows.

[jmid]

From #26089 it sounds like conflicts: "host-system-mingw" would be a reasonable annotation.
The MSys2 workflow addition in #28588 is not going to change that syslog.h is missing on Windows, and so core_unix will be unavailable on both these MinGW workflows.