ffmpeg: Update and link as console application

SDL and libcaca automatically adds `-mwindows`, treating the ffmpeg.exe as a
GUI application while at the same time making stderr go to NUL.

Found-by: Bill Westrup <wwestrup@vsscorp.com>
See: http://lists.nongnu.org/archive/html/mingw-cross-env-list/2014-12/msg00018.html
Signed-off-by: Timothy Gu <timothygu99@gmail.com>