to compile web-greeter on ubuntu you will need to:
sudo apt-get install liblightdm-gobject-1-dev gobject-introspection libgirepository1.0-dev pyqt5-dev-tools libcairo2-dev
install Python libs
sudo -H pip3 install whither
sudo -H pip3 install pygobject
then you can:
git clone https://github.com/Antergos/web-greeter.git /tmp/greeter
sudo make install
NOTE1: tested on ubuntu 18.04 using python3.6, didn't work on ubuntu 16.04 using Python 3.5
NOTE2: whither package will install Qt 5.10.1, but it has a bug QTBUG-66346 it affects Qt 5.10.1 and 5.9.4. You have to downgrade PyQt5 to 5.10.0
sudo -H pip3 uninstall PyQt5
sudo -H pip3 install PyQt5==5.10.0
You can run the greeter from within your desktop session if you add the following line to the desktop file for your session located in /usr/share/xsessions/: X-LightDM-Allow-Greeter=true.
You have to log out and log back in after adding that line. Then you can run the greeter from command line.