Installing Protocol Buffers on OpenIndiana

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:
tl;dr

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.