science/py-veusz: fix build on powerpc*

Namespace collision with LLVM:
src/threed/objects.cpp:180:24: error: expected unqualified-id

const ValVector& vec_step = stepindex==0 ? pos1 : pos2;
                 ^

src/threed/objects.cpp:188:47: error: expected expression

for(unsigned stepi=0; stepi<vec_step.size(); ++stepi)
                                    ^

src/threed/objects.cpp:191:46: error: expected body of lambda expression

pt(vidx_step) = vec_step[stepi];
                               ^

3 errors generated.