OSX requires GCC, not clang

[Forceflow]

Because of OpenMP support which clang doesn't have.

So two options:

• Wrapper script and CMakeLists editing
• Build a Trimesh2 version without OpenMP (Just stripping out the OpenMP statements in Trimesh2 src, then recompiling should work)
• In the long run: switching to ASSIMP instead of Trimesh2