Fallback when OCaml says symlinks not supported

[jonahbeckford]

• Please update master_changes.md file with your changes.

Without this PR, as a Windows user without "Run as Administrator" permission:

opam install `
  --root C:\Users\beckf\AppData\Local\opam `
  --switch C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\0\host-tools `
  Z:\source\diskuv-communicator-kernel\vendor\diskuv-ocaml\opam-dkml.opam `
  --yes --verbose --debug-level 2

...
00:19.866  SYSTEM                          install C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\0\host-tools\_opam\.opam-switch\build\opam-dkml.~dev\_build\install\default\doc\opam-dkml\README.md -> C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\0\host-tools\_opam\doc\opam-dkml\README.md (644)
00:19.866  SYSTEM                          rm C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\0\host-tools\_opam\doc\opam-dkml\README.md
00:19.868  TRACK                           after install: 7 elements, 0 added, scanned in 0.000s
00:19.868  FILE(changes)                   Wrote C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\0\host-tools\_opam\.opam-switch\install\opam-dkml.changes in 0.000s
-> installed opam-dkml.~dev
00:19.869  SYSTEM                          ln -s C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\0\host-tools\_opam\bin\opam-dkml.exe C:\Users\beckf\AppData\Local\opam\plugins\bin\opam-dkml.exe
00:19.869  PARALLEL                        Exception while computing job 788192587: install opam-dkml.~dev

#=== ERROR while installing opam-dkml.~dev ====================================#
C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\0\bin\opam.exe: "symlink" failed on C:\Users\beckf\AppData\Local\opam\plugins\bin\opam-dkml.exe: Operation not permitted

Now it succeeds:

...
00:14.605  SYSTEM                          install C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\2\host-tools\_opam\.opam-switch\build\opam-dkml.~dev\_build\install\default\doc\opam-dkml\README.md -> C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\2\host-tools\_opam\doc\opam-dkml\README.md (644)                                              ]
00:14.606  TRACK                           after install: 9 elements, 9 added, scanned in 0.001s                                ]
00:14.606  FILE(changes)                   Wrote C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\2\host-tools\_opam\.opam-switch\install\opam-dkml.changes in 0.000s
-> installed opam-dkml.~dev                                                                                                     ]
00:14.607  SYSTEM                          copy C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\2\host-tools\_opam\bin\opam-dkml.exe -> C:\Users\beckf\AppData\Local\opam\plugins\bin\opam-dkml.exe
00:14.608  SYSTEM                          copy C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\2\host-tools\_opam\bin\opam-dkml.exe -> C:\Users\beckf\AppData\Local\opam\plugins\bin\opam-dkml.exe
00:14.621  FILE(switch-state)              Wrote C:\Users\beckf\AppData\Local\Programs\DiskuvOCaml\2\host-tools\_opam\.opam-switch\switch-state in 0.010s

---

One loose end is that the if Sys.win32 then Unix.chmod dst 0o640 else (); code before the symlink is deleted (remove_file dst) was not lifted into remove_file. I wanted this PR to focus just one issue (a functioning symlink creation function).

[kit-ty-kate]

Looks good to me

[dra27]

LGTM too, thanks!