[ODE] Removing TriMesh / OPCODE support

[David Walters]

I've just submitted a change which allows all of the ODE code to
compile even if the OPCODE folder has been physically deleted, (as
long as dTRIMESH_ENABLED is set to zero obviously)

It is now possible to completely remove OPCODE / TriMesh support by
just removing the OPCODE folder from the project, and from the disk.

I felt that this simpler way of removing OPCODE if beneficial because
it doesn't involve the user removing ODE files from the project in
addition to OPCODE itself. Admittedly this isn't such a difficult
task, but now it's easier to get your compile time down if you're not
using trimeshes.

Regards,
Dave.