The `find_package()` callers expect the library name as `Potrace`, not
`POTRACE`.
Reviewed By: LazyDodo
Differential Revision: https://developer.blender.org/D8923
For work the GP team plans to land soon (T79877) potrace was taken
on as an additional optional dependency.
This diff adds building the library to the deps builder and takes
care of the integration into the build-system with the `WITH_POTRACE`
cmake switch.
Differential Revision: https://developer.blender.org/D8662
Reviewed by: brecht, sergey
