Build your own carts! (and fixes C::B build)

Answers #3247
Adds option to start building vehicles with casters, so you can build your own carts. Tested and seems to work, however there seems to be a bug in mainline Cata with both the construction menu and the vehicle examine menu, so I couldn't do any exhaustive testing.

Also fixes C::B build, since I couldn't compile this without it.