Now imagemagick migrated to Testing [1] with t64 support, maybe your patch now works for Synfig?
[1] https://tracker.debian.org/news/1525573/imagemagick-8691298dfsg1-52-migrated-to-testing/
- Bug#1063004: synfig: NMU diff for 64-bit time_t tran... Rodolfo Ribeiro Gomes
- Bug#1063004: Info received (synfig: NMU diff fo... Rodolfo Ribeiro Gomes
- Bug#1063004: synfig: NMU diff for 64-bit time_t... Adrian Bunk
