[tlbuild] xz, wget, lz4 updates

[Karl Berry]

    A totally serious question: do we still need or want to provide wget
    binaries for Darwin?

I have no opinion. I'm fine with dropping it. Or we could just leave the
current binary there, as presumably it is not hurting anything.

    What do we use lz4 for 

tlmgr uses lz4, if it's available, to compress the backups after
installing package updates, since it is fast. If it's not available,
gzip is used. If that's not available either, xz is used (which we know
is always available).

    and should any special flags be used?

Not that I know of.

    Is there a plan to switch from xz to lz4 in the future?

No.

    It might be nice to add a short section on
    https://tug.org/texlive/build.html

Yeah. I'll get around to it one of these days, maybe. --thanks, karl.