[build2] Using Symlinks in build2 Projects

[Boris Kolpackov]

We've published a note on using symlinks in build2-based projects:

https://build2.org/article/symlinks.xhtml

TL;DR: Symlinks (short for symbolic links) is a powerful mechanism for
organizing one's projects that, with a bit of effort, can be made to work even
on Windows. As a result, we consider symlinks (to both files and directories)
fair game for package development (as opposed to package consumption; see
the linked note for detail).