Created attachment 399004 [details, diff] ffmpegthumbs-4.14.3.ebuild.patch Attached patch should work.
Thanks for reporting. This is fixed in cvs. Please sync in some hours to get the changes. + + 22 Mar 2015; Johannes Huber <johu@gentoo.org> ffmpegthumbs-4.14.3.ebuild: + Migrate to USE libav by Nikoli <nikoli@gmx.us>, bug #543458. +