Add meson build support

Autotools is neither the most modern, nor the most user-friendly build system in this day and age anymore. The build fails with a very helpful "missing separator" error message if you don't have yelp-tools installed for example.

This commit makes accerciser follow the rest of the GNOME world in porting to Meson 1, a modern Python3-based build system. Configuring, building, and installing accerciser can now be done with the following commands (where _build is a randomly-picked build directory):

$ meson setup -Dprefix=/usr _build
$ meson compile -C _build
$ meson install -C _build