The script in this gist will help you buid the Google Protobuf library for use with Mac OS X and iOS. Other methods (such as homebrew or direct compilation) have issues that prevent their use. The libraries built by this script are universal and support all iOS device architectures including the simulator.
This gist was adapted from the original at https://gist.github.com/BennettSmith/7150245, and updated to deal with Xcode 7 and iOS 9, and download protobuf version 3.0.0.