ffmpeg-devel @5.0: buildbot failures for 10.9 through 10.11

[mascguy (Christopher Nielsen)]

For this new release of FFMpeg, buildbot builds are failing with two different compilation errors.

​https://ports.macports.org/port/ffmpeg-devel/builds/

For 10.11, the failure relates to __builtin_available:

libavutil/hwcontext_videotoolbox.c:420:13: error: use of unknown builtin '__builtin_available' [-Wimplicit-function-declaration]
        if (__builtin_available(macOS 10.11, iOS 9, *))
            ^

However, for 10.9 and 10.10, we're seeing the following:

libavcodec/videotoolbox.c:907:13: error: implicit declaration of function 'VTRegisterProfessionalVideoWorkflowVideoDecoders' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
            VTRegisterProfessionalVideoWorkflowVideoDecoders();
            ^

The following upstream patch may fix both issues, as they appear to be related:

​https://www.mail-archive.com/ffmpeg-devel@ffmpeg.org/msg128213.html

Need to test locally, to validate whether said patch solves the issue.