[ffmpeg] fix LGPL build, disable avisynthplus on static builds

[mcmtroffaes]

• What does your PR fix?

  Building ffmpeg[core,all] currently results in a GPL build of ffmpeg because it pulls in postproc as a default feature, which is GPL licensed, and should not be included in this build. This patch ensures that the all feature no longer pulls in the default features, thereby again allowing a pure LGPL build as intended. I've also explicitly listed postproc under all-gpl rather than relying on the default feature list.

  Whilst at it, I also noticed that the avisynthplus feature is not functional on static builds. The library itself can be built statically since 3.7.0, however there are still some bugs with this build (which I've reported upstream), but even more importantly ffmpeg is not set up to link statically against it, and can only dynamically load the avisynthplus dll. So I think it's best to disable it for now until the bugs are ironed out and ffmpeg wants to support it statically if that ever happens.

• Which triplets are supported/not supported? Have you updated the CI baseline?

  No changes.

• Does your PR follow the maintainer guide?

  To the best of my ability, yes.

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  Yes.

[NancyLi1013]

Hi @mcmtroffaes

Thanks again for your fixing. So you have tested these features, right?

[mcmtroffaes]

Yes I've tested these as usual, and in particular I checked the license of the core,all build. Before patch:

> .\vcpkg.exe install ffmpeg[core,all]:x64-windows
...
ffmpeg[all,ass,avcodec,avdevice,avfilter,avformat,avresample,bzip2,core,dav1d,fontconfig,freetype,fribidi,gpl,iconv,ilbc,lzma,modplug,mp3lame,nvcodec,opencl,opengl,openh264,openjpeg,opus,postproc,sdl2,snappy,soxr,speex,ssh,swresample,swscale,tensorflow,tesseract,theora,vorbis,vpx,webp,xml2,zlib]:x64-windows -> 4.4#6
...
> .\vcpkg.exe install ffmpeg[core,all-gpl]:x64-windows
ffmpeg[all,all-gpl,ass,avcodec,avdevice,avfilter,avformat,avisynthplus,avresample,bzip2,core,dav1d,fontconfig,freetype,fribidi,gpl,iconv,ilbc,lzma,modplug,mp3lame,nvcodec,opencl,opengl,openh264,openjpeg,opus,postproc,sdl2,snappy,soxr,speex,ssh,swresample,swscale,tensorflow,tesseract,theora,vorbis,vpx,webp,x264,x265,xml2,zlib]:x64-windows -> 4.4#6
...

Note the core,all pulling in gpl.

After patch:

> .\vcpkg.exe install ffmpeg[core,all]:x64-windows
...
ffmpeg[all,ass,avcodec,avdevice,avfilter,avformat,avresample,bzip2,core,dav1d,fontconfig,freetype,fribidi,iconv,ilbc,lzma,modplug,mp3lame,nvcodec,opencl,opengl,openh264,openjpeg,opus,sdl2,snappy,soxr,speex,ssh,swresample,swscale,tensorflow,tesseract,theora,vorbis,vpx,webp,xml2,zlib]:x64-windows -> 4.4#7
...
> .\vcpkg.exe install ffmpeg[core,all-gpl]:x64-windows
...
ffmpeg[all,all-gpl,ass,avcodec,avdevice,avfilter,avformat,avisynthplus,avresample,bzip2,core,dav1d,fontconfig,freetype,fribidi,gpl,iconv,ilbc,lzma,modplug,mp3lame,nvcodec,opencl,opengl,openh264,openjpeg,opus,postproc,sdl2,snappy,soxr,speex,ssh,swresample,swscale,tensorflow,tesseract,theora,vorbis,vpx,webp,x264,x265,xml2,zlib]:x64-windows -> 4.4#7
...

Note core,all no longer pulling in gpl, and the core,all-gpl is as before.

[ras0219-msft]

This LGTM, thanks for the fix!

[mcmtroffaes]

Great, thanks for merging!

[rubu]

@mcmtroffaes not sure if I am missing something (like options when invoking vcpkg), but currently it seems to me that this breaks the FindFFMPEG.cmake file in the package, since it still contains

if(ON)
  FFMPEG_FIND(libpostproc   postproc   postprocess.h)
endif()

and thus fails. But I have no idea how to expose the features from build to that file to fix this if.

To elaborate, I simply did vcpkg install ffmpeg:x64-windows:

and it included postproc in the output but it was not actually built

[mcmtroffaes]

@rubu Currently ffmpeg is at version 4.4 on vcpkg but your log shows 4.2 so your vcpkg must be rather outdated. I would think that updating your vcpkg to the latest master should fix your issue. The problem is that postproc depends on the gpl feature. I checked the json and this is correctly represented:

> ./vcpkg install ffmpeg:x64-windows
Computing installation plan...
The following packages will be rebuilt:
    ffmpeg[avcodec,avdevice,avfilter,avformat,core,gpl,postproc,swresample,swscale]:x64-windows -> 4.4#12

But indeed, if you try to build ffmpeg with --enable-postproc but without --enable-gpl, then indeed the ffmpeg configure script will refuse to build postproc (because it prioritizes the licensing option). This seems to be a bug on older versions of vcpkg.

[rubu]

@mcmtroffaes thanks for the response, again, since I use vpckg rarely, I kind of thought that maybe I need to update the ports (but I couldn't explain why then I should have this change, if I am on an old port). Thanks for explaining.