From 'git diff v9.12..v10.1 -- configure' + --enable-libwavpack enable wavpack encoding via libwavpack [no] + --enable-libwebp enable WebP encoding via libwebp [no] + --disable-avx2 disable AVX2 optimizations ffmpeg ebuilds now have these USE flags, may be they should be in virtual/ffmpeg too?
Might be a good idea indeed.
This was fixed months ago: http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/media-video/libav/libav-9999.ebuild?r1=1.65&r2=1.66