Use the "debian/clean" file instead of overriding the "clean" target in "debian/rules" and cleaning up manually.
Make sure the package compiles with both Libav 10 (in Debian testing and unstable) and Libav 9 (in some downstream distros like Ubuntu), the patch for Libav 10 compatibility (in commit 7bf48cda) broke the compilation with older Libav versions.