What is this, exactly?

An Objective-C interface (src/runtime) for creating and reading protobuf messages.
Source for Obj-C protobuf compiler (src/compiler), which allows us to compile protobuf source (.proto) into compiled Obj-C source. This is built and installed as protoc-gen-objc.
A number of .proto files (src/proto) that define the interface we use when interfacing with Dropcam devices and certain services. These are compiled using the compiler.
A script (compile_proto.sh) which generates the compiled Obj-C source and places it in src/gen_proto.

Prerequisites

Homebrew, to install the following dependencies. Installing it to ~/homebrew is suggested. See https://wiki.corp.google.com/twiki/bin/view/Main/MacRoadWarrior#Homebrew for more details. No matter where you install it, set HOMEBREW to the root Homebrew directory.
Protobuf, to compile the Obj-C compiler. Protobuf 2.6.0 is requried and as of this writing, it is the current version provided via Homebrew.
autoconf, to run the autogen script.

brew install protobuf

brew install autoconf automake libevent

./compile_proto.sh

compile_proto.sh runs build.sh
build.sh builds protoc-gen-objc, the Obj-C protbuf compiler, and installs it in bin.
Then, compile_proto.sh runs protoc, the generic protobuf compiler provided by protobuf, using protoc-gen-objc as a plugin. All .proto source in src/proto is compiled into Obj-C, and is output to src/gen_proto.