I'm not able to reproduce this locally, could you try the following branch and see if it fixes it (for trusty)?
$ sudo apt-get build-dep lightdm
$ bzr branch lp:~robert-ancell/lightdm/lp1405349
$ cd lp1405349
$ bzr-buildpackage
And install the appropriate .deb files from ../build-area
I'm not able to reproduce this locally, could you try the following branch and see if it fixes it (for trusty)?
$ sudo apt-get build-dep lightdm
$ bzr branch lp:~robert-ancell/lightdm/lp1405349
$ cd lp1405349
$ bzr-buildpackage
And install the appropriate .deb files from ../build-area