Code review comment for lp://staging/~gilir/lightdm-gtk-greeter/fix-ubuntu

Thanks for this branch Julien!

I am wondering if we could reduce the build-dep a bit though: I am not sure why we need gtk-doc-tools. I also think the gobject-introspection dependency can be removed with a small source change: from what I understand configure.ac looks for it but it is never used. Will look into this later, I am merging your branch in first.