Switch the dbus implementation for the client from systemd to
libdbus. The main reason is that, in flatpaks systemd is not easily
available. No phenomenological change for users of the library,
hopefully.
In order to be safe for future ABI changes and to fulfill packaging
requirements for e.g. Fedora, provide libtool like versioning for
both libraries, i.e. libgamemode and libgamemodeauto.
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>
