[poppler] cmake: gtk-doc support

[Albert Astals Cid]

So gtkdoc upstream seems to have cmake support, so I *almost* made it work

But it's obviously failing to pick up some stuff since the generated files 
look wrong.

See 
http://i.imgur.com/gdSH1pI.png
vs
http://i.imgur.com/5QSy6oP.png

Patch attached.

Can anyone try to have a look? Or anyone knows who could i ask for help on the 
glib/gnome side? I've sank a few hours into this and not sure i can move it 
forward without further help.

Cheers,
  Albert
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