The following builds RebbleOS on macOS:
git clone https://github.com/ginge/FreeRTOS-Pebble.git
cd FreeRTOS-Pebble
git submodule update --init --recursive
make
The Pebble SDK is a prerequisite for portions of RebbleOS. The
SDK is available at https://developer.pebble.com/sdk/download/.
On macOS you can install the SDK with brew
brew install pebble/pebble-sdk/pebble-sdk
If you don't use brew (or prefer not to) you can install the SDK manually.