#48 Cinelerra 7, fix for compiler warnings in ffmpeg3

[Mat Nieuwenhoven]

System Mint 18.3 (base: Ubuntu 16.04), 64 bit, AMD cpu, compiler 5.4.0 . The compiler gives warnings on a number of files about "implicit declaration of built-in function". This patch fixes those. Apply as "patch -p2 <libmpeg3_a.patch" in the ffmpeg3 directory.