[build2] Disabling rpath linking for all update operations

[Boris Kolpackov]

Boris Kolpackov <boris at codesynthesis.com> writes:

> But I agree it may be the most practical in the circumstances. So I
> will look into adding the ability to disable rapth.

I finally got around to this:

https://git.build2.org/cgit/build2/commit/?id=39a17ba7ef506192218f24047f129cb635efb02b

>From the commit message:

Specifically, the new config.bin.rpath.auto variable can be used to disable
automatic addition of prerequisite library rpaths, for example:

$ b config.bin.rpath.auto=false

Note that in this case rpath-link is still added where normally required
and for targets that support it (Linux and *BSD).

The new config.bin.rpath_link and config.bin.rpath_link.auto have the same
semantics as config.bin.rpath* but for rpath-link.

Let me know if there are any issues.