You'll need the following dependencies:

• gettext (provides msgfmt)
• gnome-settings-daemon >= 3.27
• libgdk-pixbuf2.0-dev
• libgnome-desktop-3-dev
• libgranite-dev >= 5.5.0
• libgtk-3-dev
• libhandy-1-dev >= 0.90.0
• liblightdm-gobject-1-dev >= 1.30.0
• libmutter-13-dev
• libx11-dev
• meson
• valac

Run meson to configure the build environment and then ninja to build

meson build --prefix=/usr
cd build
ninja

To install, use ninja install

sudo ninja install

Run LightDM in test mode with Xephyr:

lightdm --test-mode --debug

You can then find the debug log in ~/.cache/lightdm/log