liblightdm-qt5-3-dev software package provides LightDM Qt 5 client library (development files), you can install in your Ubuntu 14.04 LTS (Trusty Tahr) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install liblightdm-qt5-3-dev
liblightdm-qt5-3-dev is installed in your system.
Make ensure the liblightdm-qt5-3-dev package were installed using the commands given below,
$ sudo dpkg-query -l | grep liblightdm-qt5-3-dev *
You will get with liblightdm-qt5-3-dev package name, version, architecture and description in a table
| Distro | Ubuntu 14 04 lts trusty tahr |
| Section | Library development |
| Package | liblightdm-qt5-3-dev |
| Version | 1.10.0-0ubuntu3 |
| Description | LightDM Qt 5 client library (development files) |
| Sub Section | libdevel |