You're receiving this bug because the package in Summary has produced _FORTIFY_SOURCE related warnings indicating the presence of a sure overflow in a static buffer.
Even though this is not always an indication of a security problem it might even be. So please check this out ASAP.
By the way, _FORTIFY_SOURCE is disabled when you disable optimisation, so don't try finding out the cause using -O0.
Your friendly neighborhood tinderboxer
Created attachment 249712 [details]
+*xephem-3.7.4-r1 (11 Oct 2010)
+ 11 Oct 2010; Kacper Kowalik <email@example.com> -xephem-3.7.2.ebuild,
+ xephem-3.7.3.ebuild, -xephem-3.7.4.ebuild, +xephem-3.7.4-r1.ebuild,
+ Fix buffer overflow wrt bug 339901. Thanks to Diego for the report.
+ Respect CC. Drop old. Add explicit RDEPEND=