ffmpeg depends on libgcc_s_seh.dll when CMAKE > 3.8.2 is called by ffmpeg-cxc-mingw (mingw64 cross compile). Downgrading CMAKE relieves the dependency.
This issue no longer appears to be present when CMAKE >= 3.16.1 is used for the mingw64 cross compile. In short, use CMAKE <= 3.8.2 || CMAKE >=3.16.1 to avoid the generation of the libgcc_s_seh.dll dependency.
ffmpeg depends on libgcc_s_seh.dll when CMAKE > 3.8.2 is called by ffmpeg-cxc-mingw (mingw64 cross compile). Downgrading CMAKE relieves the dependency.