I should add that even on OSX, the Keyboardio firmware is built with ‘genuine’ GCC. Arduino includes an avr-gcc ‘cross-compiler’ which knows how to output machine code for the chip that powers the Model 01. The system compiler (clang) builds binaries that will run on your Mac, but doesn’t know how to compile to avr.
One other option for getting up and running is Feedback requested: simplified software installation – As of now, this may not get you commandline builds, but should get you to building and flashing firmware much more quickly from within the Arduino IDE