[Question] Show build-ffmpeg location of my local ffmpeg source folder

[someziggyman]

Is there a correct way to edit build-ffmpeg for it to build ffmpeg using source files in my local folder?

Reason:
some VTT subs have these "REGION" and "STYLE" values in the begginign of the file that need to be ignored in order to be correctly converted to SRT or other formats. Otherwise the resulting output are just zero byte files. It seems to be a known but, but official ffmpeg repo owner just ignores the thing. https://ffmpeg.org/pipermail/ffmpeg-devel/2022-May/296353.html

More about the issue here:
yt-dlp/yt-dlp#3622 (comment)

So, I edited webvttdec.c file, downloaded ffmpeg snapshot source and replaced the file there. Now I'l like build-ffmpeg to use this local source copy of mine. Is that possible or am I over complications things? :-)