xbps-src: binpkgs to resolve builddeps

[Juan RP]

Hi,

I've just added code to xbps-src that makes it possible to install
binary packages to resolve build dependencies.

That means that if for example you're going to build vlc, and some
required build dependencies are available as binary packages in
XBPS_PACKAGESDIR, they will be installed in the chroot automagically.
If a required build dependency didn't match a binary package, it
will be built from source.

To use this feature you'll need xbps utils >= 20100114 from bzr
head branch and the XBPS_PREFER_BINPKG_DEPS set in xbps-src.conf.

With this feature you can rebuild a single pkg faster and much
easier than before, and if you wish you can destroy the chroot
once it's built (if you don't need it again).

Enjoy!