build: add "git-dist" target for making gzipped tarballs
Since MANIFEST is tied to files tracked by git, adding generated
files such as NEWS to that is more effort than its worth (esp.
when I'm wondering if MakeMaker is useful compared to only using
GNU make).
I also have trouble reading CamelCase, so lower-case names
are nicer and more consistent with previous releases
(which were all generated with "git archive"); but did not
include NEWS.