From the OEM for the polycarbonate plastic used in the process: weld-on 45 or Methylene Cloride (cancerous) (dissolve good quantity of plastic into the solvent until it is thick and can be used as a glue) I think I'd rather go for weld-on 45, it can be had for $5 and won't give you cancer.
I would also be interested in an european alternative. I've got some original (barely used) CD's from the mid 90s which have these cracks..
I personally use Locktite 414, I spread both layers of the cracked disc with a toothpick and apply it gently in between.