To get everything to work, I had to change the sed line to this:
cd /tmp/${_pkgbase} && sed -i -e 's/lua52/luajit/g' -e 's/52arch/luajit/g' -e 's/ffmpeg-git/ffmpeg/g' -e "s/-Dlua='auto'/-Dlua='luajit'/g" PKGBUILD

Maybe consider publishing this package to the AUR?