While revisiting Mosh, I came across an issue I didn't remember when going through my initial Moshtribulations. Trying to compile the Protocol Buffers failed with the following error due to a missing package:
google/protobuf/text_format.cc:36:18: math.h: No such file or directory
The missing package I needed to install was header-math:
pfexec pkg install header-math
After that the compile was fine.