Hello,
I use Xubuntu 16.04 LTS. Yesterday the update manager updated several *libqt5* libraries. Today I try once again to compile the official 2.6.20 and ... it is successful ! I have all the *.debs in the directory.
I think, but this is only a guess, that these updates from Canonical have "cured" the compilation process.
The qt5 update as such (which by the way should have fixed the old issue with appmenu-qt5 and keyboard shortcuts (http://avidemux.org/smif/index.php/topic,16956.0.html)) has for sure nothing to do with the successful compilation of Avidemux on your system. Some files, damaged due to lurking hardware issues, might have been rewritten in course of the update though.