To ease development, create a gamemode.pc and a gamemode-auto.pc
file, that other projects can use integrate with gamemode. The
former if they want to integrate at the source level and the latter
if the automatic integration is preferred.
This also explicitly constructs reusable library components to make it
easier to extend the project over time. Notably we make less assumptions
about the host system and use pkgconfig where appropriate to give us
system details, such as the systemd system unit directory for packaging.
This can be overriden with the meson option.
Signed-off-by: Ikey Doherty <ikey@solus-project.com>
