UbiCast FFmpeg build

Build the ffmpeg-ubicast package using an artifact from FFmpeg-Builds.

Update

To update FFmpeg, follow this process:

• Get the latest FFmpeg version number in the official site.
• Get the latest build named Auto-Build <date> (do not pick Latest Auto-Build (<date>) because artifact links are not perennial).
• In the artifacts, search the one for Linux 64, licensed as GPL and for the targetted FFmpeg version. Example: https://github.com/BtbN/FFmpeg-Builds/releases/download/autobuild-2026-03-30-13-13/ffmpeg-n1.2-7-ga3475e2554-linux64-gpl-1.2.tar.xz.
• Update the version in debian/pkgvars.
• Update the artifact link in debian/rules.
• Commit the change with a comment like: Update FFmpeg to version 1.2 | #1234.