Many changes since the 3.3x series. See http://syslinux.zytor.com/history.php for more details.
Just copy syslinux-3.36.ebuild to syslinux-3.51.ebuild and all is good.
*** Bug 191312 has been marked as a duplicate of this bug. ***
24 Oct 2007; Steve Dibb <beandog@gentoo.org> +syslinux-3.51.ebuild, +syslinux-3.52.ebuild: Version bump