The configure of your package checks for flex, even though your package might not contain .l files that flex uses to generate parsers. Your ebuild does not list a dependency on flex as per http://tinderbox.dev.gentoo.org/misc/dindex/sys-devel/flex .
Either disable the flex test during configure to avoid false positive, or add the dependency upon flex if it's needed by your package.
Thanks in advance,
11 May 2008; Markus Meier <email@example.com> graphviz-2.12.ebuild,
graphviz-2.16.1.ebuild, graphviz-2.16.1-r4.ebuild, graphviz-2.18.ebuild:
add missing flex dependency, bug #221391
Thanks for the report.