Follow kos-setup-easy.md first.
sudo apt install unrar
git clone https://github.com/ianmicheal/SDL-dreamhal--GLDC.git
cp "SDL-dreamhal--GLDC-main/Precompiled libs/*.a" "/opt/toolchains/dc/kos/addons/lib/dreamcast/"
unrar x "SDL-dreamhal--GLDC-main/Precompiled libs/include.rar" "/opt/toolchains/dc/kos/addons/include/"