It seems the build works except with Mac. Can someone that regularly
builds on Mac help? It is probably just a matter of introducing #include
<Particle.h> in the right place - it was previously included through
<Model.h> (and it doesn't have to). I don't know why it's Mac specific.
It's not Mac specific. That machine builds with per-cpp compilation. So
you can replicate it on any machine by setting IMP_PER_CPP_COMPILATION
to include the module that isn't building.