Explore using a build system for generating data files

There is a number of various data files under data/ in the source tree. Those are related to systemd, udev, environment, dbus, and so on. The files are generated by invoking the Makefile in the data directory and overriding variables in the command line. Examples:

https://github.com/snapcore/snapd/blob/dd2b0a319ee8f07ae356fd3da62263054ebef3b9/packaging/fedora/snapd.spec#L446

or

https://github.com/snapcore/snapd/blob/dd2b0a319ee8f07ae356fd3da62263054ebef3b9/packaging/arch/PKGBUILD#L63

We already use autotools as a build system for cmd/ and the packaging does set the relevant configure options in arguments to configure. Additionally, the installation of data files happens as a separate step.

Explore whether it would be possible to reuse autotools (or any other common build system, provided it’s used in the rest the codebase) for generating and installing data files.