It turns out to be inconvenient to delete the downloaded datafiles with
distclean, so I propose a new target which does that instead.
The closest conventional target is 'maintainer-clean'; the difference
here is that having the original source tarball is not enough to
reconstruct these files.
The checksum file is used by the test infrastructure to verify the downloaded
test database is the one we had in mind. Note that this test is
rather strict, and the the checksum file needs to be recommitted when
the database is regenerated.
add a pattern .gitignore to ignore the actual databases
