This repository is used as a submodule for the main OpenWebRTC repository. It contains scripts to bootstrap a build environment, build dependencies and also the build system infrastructure.
You don't need to clone it separately to the openwebrtc repository. If you cloned the openwebrtc repository recursively then you already have this in the openwebrtc/scripts/ subdirectory. If you cloned openwebrtc without the --recursive option, you may need to run:
cd openwebrtc
git submodule init
git submodule update
...and then you will have the desired setup.
Build instructions are found here.