To install the agent you have to add io.github.koko-ng.geoclue-agent in /etc/geoclue/geoclue.conf in the whitelist field, under [agent].
To compile on elementary os you have to install some pacakges first: sudo apt install cmake libgeoclue-2-dev valac libdbus-glib-1-dev libglib2.0-dev libgtk-3-dev.
Then, cmake . && make && sudo make install, you need to log off, log in after that. I even recommend you to reboot.