configure: reject older libswresample

mpv still builds with ffmpeg 1.0.x, however libswresample keeps cuasing
trouble. In older releases, libswresample simply crashed when
downmixing. In somewhat newer versions, it produces distorted output and
downmixing isn't even close to correct.

With ffmpeg release 1.1 (ffmpeg git tag n1.1), everything seems to work
fine. The release uses 0.17.102 as libswresample version, so bump the
required minimum version to that.