the resulting tarball is available and was prepared as follows
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
git clone --recursive git://repo.or.cz/syslinux.git
apply the debian patchen
cd syslinux/
for patch in ../debian/patches/*.patch; do
echo PATCH $patch
patch -p1 < $patch
echo
done; unset patch
grab the shortened commit hash from there
git log -1 | head -1
cd ../
e.g. as of Aug 2021
mv syslinux/ syslinux-05ac953/
tar czf syslinux-05ac953.tar.gz syslinux-05ac953/
while possibly getting rid of EFI
cd syslinux-*/
mv Makefile Makefile.dist
sed -r 's/^all_firmware := bios .*/all_firmware := bios/' Makefile.dist > Makefile
eventually attempt to build it right away
make
clean-up everything and deliver the updated tarball
rm -rf debian/ syslinux_6.04~git20190206.bf6db5b4+dfsg1-3.debian.tar.xz syslinux-05ac953/
mv syslinux-05ac953.tar.gz ../slackbuilds/syslinux-next/
http://cdn.kernel.org/pub/linux/utils/boot/syslinux/