[build] ship booth.pc with basic booth build information for downstream packages to use

some packages, such as pcs, needs to have specific information about booth
installation paths in order to configure booth properly.

in a similar fashion as systemd (shipping systemd.pc), add similar feature to booth.

In general this change does NOT affect any downstream distributions as they all
use pretty much standard paths, but it makes it a ton easier to build / install
and test booth in CI where all package are installed in non-standard paths.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>