So after trying to clean up the code in the Akonadi Resource it seems like libmapi++ is not going to be a header-only library much longer.
Either way, the main reason for keeping it that way was to make it easier for other people to use. Now that libmapi++ is part of the openchange build-system, and it's distributed as part of the openchange distribution, that is not really needed anymore, because it gets "built" anyway.
However, that's going to be in the near-future. First I want to concentrate in getting the resource stable and working with the current version of akonadi and whatever client is able to access it (which I'm still trying to figure out).
After a quick trial, Mailody seems not to be very up-to-date with Akonadi development. So right now I'm trying to figure out what to use to test the resource.