[ODE] Convex Primitive code added

[Rodrigo Hernandez]

I have added the convex primitive code to unstable, along with some 
small fixes to autotools,
So, those who wanted to take a look at the code can do so now :)

I made changes to test_boxstack in order to test the convex object, 
please refer to its code to see an example of
usage, although only 2 functions are made available to the end user, 
setting the convex data may be tricky
(you have to specify a collection of planes, a vertex buffer and a kind 
of index buffer).

Anyway, If you have questions feel free to ask :)

Oh! and one more thing, the new code has been surrounded by

//--> Convex Collision
//<-- Convex Collision

comments, look for those if there is a need to #ifdef the code.

Cheers!