docs package has been removed without explanation, that should probably be restored. There's also ABI removals so you need to basically rebuild everything against this and validate the changes. We also don't do 80 columns and use Markdown in changelogs.
No you should be using the reconfigure macro.
Why are you doing this in a build dir now?
Their change to enforce separate build directories is dumb, their reasoning is dumb (basically "another build system we're not even using does this, so let's do it too"), so let's revert it. Please revert the changes based in https://github.com/libvirt/libvirt/commit/f96395e78eaccffbf128336382c74b1250f04032 in a patch. Also you didn't address my issues with the summary or the docs.