A follow-up to !7947 (merged). Following a suggestion of @doctormo, add a submodule src/3rdparty/capypdf pointing directly at upstream, and some CMake integration around it. Thus PPA builders can now i) access the capypdf source code, and ii) build it since they no longer need meson >=1.10 (or at all).
This is Draft because I haven't yet managed to test that these changes actually work on launchpad.net. That will be done as soon as I figure out how to get launchpad.net to not be a completely broken mess.