Oh, sorry!

It seems you put binary ffmpeg snapshot!

Try to search for ffmpeg-5.1.tar.xz and check it actually source tarball....

чт, 28 июл. 2022 г., 00:11 Andrea paz <gamberucci.andrea@gmail.com>:

>You re-run ./autogen.sh after editing configure.ac and it finished ok?
Yes, I had already done that
> Is there unpacked ffmpeg-5.1 source now in thirdparty?
Yes, tarball unpacked