Fix some oversights in distprep and maintainer-clean targets.

At least two recent commits have apparently imagined that a comment in
a Makefile stating that something would be included in the distribution
tarball was sufficient to make it so.  They hadn't bothered to hook
into the upper maintainer-clean targets either.  Per bug #5923 from
Charles Johnson, in which it emerged that the 9.1alpha4 tarballs are
short a few files that should be there.