Install Retro68, then set $RETRO68_TOOLCHAIN_PATH
to your toolchain path and $RETRO68_INSTALL_PATH
to the path of the cloned repo, then run the following commands:
mkdir -p build
cd build
cmake .. -DBUILD_QJS_LIBC=OFF -DRETRO68_ROOT=$RETRO68_TOOLCHAIN_PATH -DCMAKE_TOOLCHAIN_FILE=$RETRO68_INSTALL_PATH/cmake/retroppc.toolchain.cmake.in
make -j$(nproc)
The resulting file will be the stuffit file os9fetch.bin