I installed ffmpeg 0.8.9 on ubuntu11 by
./configure --enable-gpl --enable-nonfree --enable-pthreads --enable-libfaac --enable-libmp3lame --enable-l
I am using Ubuntu 11.04, and I also came across this error - Unknown encoder 'libx264'. Installing the package libavcodec-extra-53 resolved the problem.
Unknown encoder 'libx264'
libavcodec-extra-53
In Ubuntu 14.04 LTS the package that is needed is called libavcodec-extra-54 (and libav-tools)
libavcodec-extra-54
libav-tools