Update URL for mingw sourceforge repositories.

These repository URLs currently return 301, and the script bails out rather than following the redirect. After updating the URLs (and temporarily tweaking cross_compile_ffmpeg.sh so it would use the patch from the currently checked-out repo, rather than fetching the latest from your master branch), I successfully built an x64 ffmpeg on a fresh Debian VM.

(I had to install cmake and yasm from unstable to build successfully; the versions that come with wheezy are too old.)