Does not build with FFmpeg 5.x / 6.x

[brad0]

Trying to update our OpenBSD port / package to FFmpeg 6.1 we ran into Mediastreamer2 which currently does not build against FFmpeg 5.x / 6.x API.

FAILED: src/CMakeFiles/mediastreamer2.dir/utils/ffmpeg-priv.c.o
/home/brad/ports/pobj/mediastreamer2-5.3.41/bin/cc -DBCTBX_LOG_DOMAIN=\"mediastreamer\" -DHAVE_CONFIG_H -DMS2_EXPORTS -DMS2_FILTERS -DMS2_INTERNAL -DQRCODE_ENABLED -DVIDEO_ENABLED -D_TRUE_TIME -Dmediastreamer2_EXPORTS -I/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/include/OpenGL -I/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/include -I/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/audiofilters -I/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/utils -I/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/voip -I/home/brad/ports/pobj/mediastreamer2-5.3.41/build-aarch64 -I/home/brad/ports/pobj/mediastreamer2-5.3.41/build-aarch64/src -isystem /usr/local/include -isystem /usr/X11R6/include -O2 -pipe -DNDEBUG -fPIC  -Wall -Wuninitialized -Qunused-arguments -Wno-unused-function -Wno-array-bounds -Wstrict-prototypes -Wno-error=strict-prototypes -MD -MT src/CMakeFiles/mediastreamer2.dir/utils/ffmpeg-priv.c.o -MF src/CMakeFiles/mediastreamer2.dir/utils/ffmpeg-priv.c.o.d -o src/CMakeFiles/mediastreamer2.dir/utils/ffmpeg-priv.c.o -c /home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/utils/ffmpeg-priv.c
/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/utils/ffmpeg-priv.c:26:14: warning: call to undeclared function 'avcodec_encode_video'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        int error = avcodec_encode_video(avctx, avpkt->data, avpkt->size, frame);
                    ^
/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/utils/ffmpeg-priv.c:41:2: warning: call to undeclared function 'avcodec_get_context_defaults'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        avcodec_get_context_defaults(s);
        ^
/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/utils/ffmpeg-priv.c:45:17: error: conflicting types for 'avcodec_alloc_context3'
AVCodecContext *avcodec_alloc_context3(AVCodec *codec) {
                ^
/usr/local/include/libavcodec/avcodec.h:2306:17: note: previous declaration is here
AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);
                ^
/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/utils/ffmpeg-priv.c:46:9: warning: call to undeclared function 'avcodec_alloc_context'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        return avcodec_alloc_context();
               ^
/home/brad/ports/pobj/mediastreamer2-5.3.41/mediastreamer2-5.3.41/src/utils/ffmpeg-priv.c:46:9: warning: incompatible integer to pointer conversion returning 'int' from a function with result type 'AVCodecContext *' (aka 'struct AVCodecContext *') [-Wint-conversion]
        return avcodec_alloc_context();
               ^~~~~~~~~~~~~~~~~~~~~~~
4 warnings and 1 error generated.
ninja: build stopped: subcommand failed.