I think you need not just   --with-ffmpeg-git, but 

--with-git-ffmpeg=https://git.ffmpeg.org/ffmpeg.git

But be aware about ffmpeg.git patch 2 not yet updated  (so you better to rename it to something not ending with .patch, so build Will skip it.. )

Or try to adapt it piecewise.. (patch modifies mpegts muxer for blu-ray compatible muxing...) 

In my slackbuild i have this line moving this patch out of the way:

 # ffmpeg.git changed mpegts muxer again - 26/08/2020
mv thirdparty/src/ffmpeg.git.patch2 thirdparty/src/_ffmpeg.git.patch2



On Wednesday, April 28, 2021, Andrea paz via Cin <cin@lists.cinelerra-gg.org> wrote:
With the release of FFMPEG 4.4 I tried to compile CinGG with git-ffmpeg.
I used the following command from ./configure:

./configure --with-single-user --with-git-ffmpeg --with-booby

And then I compiled with:

make 2>&1 | tee /tmp/cin5.log && make install

The compilation crashes immediately with the cin5.log that I attach.

Do you have any advice for me or is this an impossible task for a
non-programmer?