[tlbuild] lz4?

[Karl Berry]

We are considering switching to using lz4 (lz4.org) to make the local
backups for tlmgr updates, instead of xz. lz4 is much, much, faster, and
maximum compression is not required for local files. Of course if lz4 is
not available, we'll just fall back to xz as now.

Apparently the place to get the code is here:
https://github.com/lz4/lz4/releases/latest

There is just a Makefile (which appears to assume GNU make),
no configure. It already tries to maximally optimize.
The result is a binary ./lz4. There is no need to install it anywhere.

If we go forward, I suppose we'll install this in tlpkg/install/lz4, but
I haven't created it yet and munged the tlpsrc files. So I guess just
send the (gzipped please :) binaries to me or Norbert.

There is no hurry. Mostly we're wondering if it is a viable idea.
--thanks, karl.