[tlbuild] building / including xz into the build process

[Karl Berry]

    would be rather easy to integrate into the TL2009 build system, 

Thanks for offering, but please don't.  It should remain completely
separate.

    Maybe disabled by default like xindy (except for a native-texlive-build).
    Karl?

xz binaries should never end up in the normal TL bin directory.  Chance
of confusion and mistakes is too high.  We don't want to be distributing
a user-facing xz.  We only include the binaries "hidden" inside the
installer directory.  It's in our source tree mainly for convenience,
and should only be built "by hand" (once).

As Norbert said, we do eventually need xz and xzdecode binaries for all
platforms.  I expect we'll be ready to start checking them in within a
couple more days.  Will write about it again then ...

Thanks,
Karl