[packagekit] Switching to Meson

[Richard Hughes]

Hi all,

Like a lot of other projects, PackageKit has just switched from
automake to meson as a build system. Most of the work was done by
Corentin Noël and we got a lot of help from Martin Blanchard too, and
I fixed up several of the tests and assumptions that seemed to break
when switching build systems. We've now tested the new meson build
system in quite a lot of different architectures and with a few
different backends, and it all seems to work well.

Please, check out master and try to compile your usual backend and
compile options and tell us (preferably with a PR :) if we broke
anything. I've also used the branching opportunity to remove some of
the dead backends and also the GTK2 module helper. If anyone else sees
any other nice cleanups or dead wood, please let me know.

Full disclosure: this is not me "restarting the project" or part of
any grand plan, but I think the meson work is important to keep the
project building and to keep the life support machine turned on.
Comments welcome.

Richard.