Switch to using xz for dist

The automake 'distcheck' target accidentally passes the '--best' arg to gzip when decompressing, when it only makes sense for compressing. This is harmless with the normal gzip impl, but triggers an error with the busybox gzip.

We have been relying on getting 'gzip' indirectly via an existing pkg dependency, and recently this changed from the full gzip to a busybox minimal gzip in openSUSE tumbleweed.

While we could add an explicit gzip dep, we instead take the opportunity to switch the project over to 'xz'.