The following builds RebbleOS on macOS:
git clone https://github.com/pebble-dev/RebbleOS.git
cd RebbleOS
git submodule update --init --recursive
make
The Pebble SDK is a prerequisite for portions of RebbleOS. The
SDK is available at https://developer.rebble.io/developer.pebble.com/sdk/download/index.html#mac-os-x.
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.