Add support gst-libav build on MSVC using downloaded prebuilt ffmpeg

Submitted by Seungha Yang

Link to original bug (#797315)

Description

On windows, installing ffmpeg and building gst-libav requires more
effort compared to *nix environment. This patch will make it easier
on Windows MSVC build. (but user should add ffmpeg's dll path to
%PATH% env. manually when 'uninstalled' environment is required)