keyword-generation.sh: Suppress empty variables.

* keyword-generation.sh: Don't output variables with an empty list
of functions.
Keywords regenerated from eclasses.

* ebuild-mode-keywords.el (ebuild-mode-keywords-*): Regenerated
from eclasses.
Use xz for compression of tarball.

* Makefile (dist): Use xz for compression.
(clean): Also remove *.xz.
Version 1.21 released.