I've tested that ulex builds (and works) on OSX, and see no reason
it would not do so on other platforms, so I'm lifting the linux
restriction in platforms.
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
