To generate `sources.txt`, first run configure ffmpeg
../src/configure
Then, generate `vars.txt` from config.asm
awk '{print $2}' config.asm >vars.txt
Build `sources.txt` from vars.txt and ffmpeg's `Makefile`s
make -f sources.mk >../sources.txt
Finally, clean up what `./configure` left behind
git clean -df .