Comment 3 for bug 1317131

To be honest, I don't understand it either. I made x264 packages of the current version with ffms and lavf support. You can find them in my VLC PPA (be aware that a slightly newer version of libav/ffmeg will be installed through the dependencies).
As for the avisynth support, it requires vapoursynth or avxsynth to be installed, which is not in the Ubuntu repos.

PS: *.y4m is a raw YUV image format. It's not intended to be saved as a video file, because it's hueg, but you can pipe the output to a command line tool like ffmpeg or x264:
wine avs2pipemod.exe -y4mp "./test.avs" | x264 --preset slower --crf 17 -o "./neu.mkv" - --demuxer y4m