Blender Git Loki

Fix for bug #4425:
- Assert was causing blender to crash on non-manifold meshes with a decimation
modifier. Even in a debug build, there's no reason to crash blender then,
because the modifier handles this ok and displays a warning message.

Part of the problem is NDEBUG not being set in scons/linux for release builds,
but always enabled for makefiles even in debug builds.