Hello,
there has been a bug report for the package of Avidemux 2.6.3 on getdeb.net which I published
that the x264 option is missing.
I did some research and have found out that it is only missing in the GTK GUI but exists in the QT GUI.
Also the build log always says that x264 is enabled:
http://build.getdeb.net/build/logs/avidemux2.6_2.6.3-1~getdeb2~quantal-amd64-20130327-1849 (http://build.getdeb.net/build/logs/avidemux2.6_2.6.3-1~getdeb2~quantal-amd64-20130327-1849)
This is the bug report:
https://bugs.launchpad.net/getdeb.net/+bug/1167673 (https://bugs.launchpad.net/getdeb.net/+bug/1167673)