To generate the Slackware ffmpeg package for XBMC use:
(We assume the xbmc source is compiled on /tmp)
1. Copy the ffmpeg.SlacBuild file in /tmp/xbmc-xx.x/lib/ffmpeg/
2. Copy slack-desc in /tmp/xbmc-xx.x/lib/ffmpeg/
3. cd into /tmp/xbmc-xx.x/lib/ffmpeg/
4. run ./ffmpeg.SlackBuild

The Slackware package will be generated in /tmp