matita and ulex must build against the same version of camlp5, so in an attempt to force them to always be the same I am adding a propgatedBuildInput to ulex.
Granted Matita still requires camlp5_traditional and this is less obvious in the matita file now, so I am not entirely sure this is the right design choice.
svn path=/nixpkgs/trunk/; revision=30552