To build:
git clone --recurse-submodules [email protected]:jasondbiggs/RectanglePacking.git
cd rectanglepacking/
mkdir build
cd build
cmake -DWolframLanguage_INSTALL_DIR=/Applications/Mathematica.app/Contents -DJLLU=4 ..
make -j4 install
Then run PacletDirectoryLoad in WL on the appropriate directory.