You should add required libraries to the HACKING file

I think this is pretty bad practice around many open source codebases, not to include the list of dependencies. I am of opinion this is about the most off-putting part for new developers trying to join the project.

Something like a

    sudo apt-get install valac ...

line in HACKING file would go a long way (I'm pretty sure vast majority of people are on a Debian derivate nowadays and this being a Launchpad hosted program means it's something like 99% more likely).