ffmpeg: Set license to GPLv3

The Zeranoe builds linked on ffmpeg.org have the --enable-gpl and
--enable-version3 configure flags set, so they're licensed under the
GPLv3.
targets: Add all copies to the FileWrites item list

The FileWrites item list is used by MSBuild to keep track of all files
that have been written during the build, and need to be cleaned up in
the Clean target. Adding the copied files to them ensures that they are
removed when the Clean target is run.
Merge pull request #1 from oxan/minor-fixes

Two minor fixes to existing packages (Thanks @oxan )