Andrea, that is much further then I got so I can not say if this is a workaround but if you have the time, could you look into this:

In the file cinelerra-gg-qt6/cinelerra-5.1/build/CMakeCache.txt around line 297 is the following:

    FFMPEG_INCLUDE_DIR:PATH=/usr/include/ffmpeg

check if this is the correct location for your Arch system; for my Fedora 44 this is where the 7 libraries reside (libavcodec, libavdevice, libavfilter, libavformat, libavutil, libswresample, libswscale).  BUT I do not know for sure if that is what is expected to be there.  If your PATH is different, edit the CMakeCache.txt to correct it for your system as a workaround for now.