With the genericjb use flag enabled, asterisk 1.2.11 won't build as the patch doesn't apply. It looks like it's using the wrong -P setting, as I can apply the patch fine manually (maybe you need a cd ${WORKDIR} before the epatch line?) Also, the downloaded patch differs from the one off of the Asterisk backports wiki. The original is at http://asterisk-backports.org/downloads/1.2.11-jb.patch although I'm guessing you just added some other fixes to the original one, as the Gentoo version is larger. BTW: Is there a website for Gentoo VoIP, or just #gentoo-voip on freenode?
There is just #gentoo-voip on freenode and this bug is outdated now as the ebuild is no longer in tree