[Libguestfs] [PATCH v2 REPOST] lib: Allow db_dump package to be a weak dependency

[Richard W.M. Jones]

We went around the houses a few times last year in order to try
to fix this old Debian bug:

https://bugzilla.redhat.com/show_bug.cgi?id=1409024

My last attempt was:

https://www.redhat.com/archives/libguestfs/2017-October/msg00058.html

which I believe was neither reviewed nor rejected, so I'm reposting
the same patch again, simply rebased against current git.

Rich.