Often Linux systems are shipped with wget(1) by default (and no curl).
Many BSDs, macOS, and e.g. some Linux minimal/container images
comes with curl(1) (and no wget).
Attempting to download with curl if wget is not available increases
the likelihood for this to succeed.
Many of the external links found in the notmuch source can be resolved
using https instead of http. This changeset addresses as many as i
could find, without touching the e-mail corpus or expected outputs
found in tests.
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.
