the resulting tarball is available and was prepared as follows
cd ~/slackbuilds/syslinux-next/ mkdir tmp/ cd tmp/ wget https://deb.debian.org/debian/pool/main/s/syslinux/syslinux_6.04~git20190206.bf6db5b4+dfsg1-3.dsc wget https://deb.debian.org/debian/pool/main/s/syslinux/syslinux_6.04~git20190206.bf6db5b4+dfsg1.orig.tar.xz wget https://deb.debian.org/debian/pool/main/s/syslinux/syslinux_6.04~git20190206.bf6db5b4+dfsg1-3.debian.tar.xz tar xaf syslinux_6.04~git20190206.bf6db5b4+dfsg1-3.debian.tar.xz tar xaf syslinux_6.04~git20190206.bf6db5b4+dfsg1.orig.tar.xz
apply the debian patchen
cd syslinux-6.04~git20190206.bf6db5b4/ for patch in ../debian/patches/*.patch; do echo PATCH $patch patch -p1 < $patch echo done; unset patch cd ../
deliver the updated tarball
mv syslinux-6.04~git20190206.bf6db5b4/ syslinux-6.04-debian/ tar czf ../syslinux-6.04-debian.tar.gz syslinux-6.04-debian/
http://cdn.kernel.org/pub/linux/utils/boot/syslinux/
https://packages.debian.org/unstable/syslinux
https://wiki.syslinux.org/wiki/index.php?title=Development