Windows: Prefer junctions for directory symlinks

Update the approach added by commit afb7f6e4ff (cmake: Add '-E
create_symlink' support on Windows, 2018-06-11, v3.13.0-rc1~75^2) to use
junctions, as suggested [here](https://superuser.com/a/1291446/140450).
This allows them to work under security limitations on Windows.

Fixes: #23257