ffmpeg is compiled without non-free AAC encoder.
By typing "ffmpeg" into terminal you can see what flags were enabled during
compilation.
Here is ffmpeg on my Parabola computer (on Trisquel I think libx264 isn't
available in ffmpeg due to software patents):
ffmpeg
ffmpeg version 1.2.1 Copyright (c) 2000-2013 the FFmpeg developers
built on May 10 2013 16:31:05 with gcc 4.8.0 (GCC) 20130502 (prerelease)
configuration: --prefix=/usr --disable-debug --disable-static
--enable-avresample --enable-dxva2 --enable-fontconfig --enable-gpl
--enable-libass --enable-libbluray --enable-libfreetype --enable-libgsm
--enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb
--enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus
--enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libspeex
--enable-libtheora --enable-libv4l2 --enable-libvorbis --enable-libvpx
--enable-libx264 --enable-libxvid --enable-postproc
--enable-runtime-cpudetect --enable-shared --enable-vdpau --enable-version3
--enable-x11grab
libavutil 52. 18.100 / 52. 18.100
libavcodec 54. 92.100 / 54. 92.100
libavformat 54. 63.104 / 54. 63.104
libavdevice 54. 3.103 / 54. 3.103
libavfilter 3. 42.103 / 3. 42.103
libswscale 2. 2.100 / 2. 2.100
libswresample 0. 17.102 / 0. 17.102
libpostproc 52. 2.100 / 52. 2.100
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options]
outfile}...
