nvme-cli & json vs blktests

[Christoph Hellwig]

Hi all,

I recently did a long overdue update of blktests on my main test
machine, and a large part of the nvme tests started failing.  I
eventually bisected this down to:

commit 444f0df9f0c537335f59c1721240212852656e5b
Author: Hannes Reinecke <hare at suse.de>
Date:   Wed Oct 30 15:51:45 2024 +0900

    nvme: check for controller device in _nvme_connect_subsys()


which adds a requirements for json output from nvme connect, and
just silently fails if not present.

Now after figuring this out I went to rebuild my nvme-cli after installing
the json-c development package.

... and nothing changes.

It looks like the nvme-cli build system unfortunately doesn't pick up
changes in available dependencies even after a make clean.  It turns
out a make purge eventually fixed it, but that's a little unexpected.

Can we get proper feature check for json support in blktests and maybe
make the nvme-cli build a little less arcane?