[MacPorts] #50703: ffmpeg port modifications to enable splitting off legacy port versions with minimal effort

[MacPorts]

#50703: ffmpeg port modifications to enable splitting off legacy port versions with
minimal effort
-------------------------+--------------------------------
 Reporter:  rjvbertin@…  |      Owner:  macports-tickets@…
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:  has_patch    |       Port:  ffmpeg
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 As announced elsewhere, I've implemented a proof of concept set of changes
 to the current (2.8.6) FFMpeg port so that it becomes trivial to support a
 legacy version installed in parallel to a current version. This would make
 it possible to keep the main `port:ffmpeg` at the latest/current release
 even if because of ABI changes certain dependent ports still require an
 older version.

 The principle is simple: use FFMpeg's configure options to build the
 binaries and libraries using an appropriate suffix (tentatively called
 `compat_version` in my draft) and accompanying options to ensure that
 items under ${prefix}/include and ${prefix}/share do not cause version
 conflicts.
 If the port is not configured in legacy mode, symlinks are used to make
 the relevant items available where they would be in a simple installation.
 Legacy mode is currently implemented as an inactive
 `port:ffmpeg-${compat_version}` subport, which means that dependents can
 request that compat_version explicitly.

 Thus:
 - dependents will find the main ffmpeg port and files as usual
 - dependents that need to use a legacy version only need to be patched so
 that they request the suffixed pkgconfig files (ports not using pkgconfig
 for this will take a bit more work)
 - currently installed dependents will continue to work thanks to the
 symlinks to (only) the runtime shared libraries (`libfoo.N.dylib`). This
 is a personal preference of mine; those symlinks could of course also be
 omitted in legacy mode too.

 Note that these modifications can be applied only to the v2.8.6 ffmpeg
 port; it should be able to co-exist peacefully with a v3.0.0 ffmpeg port
 built normally. I'd advise building all ffmpeg ports like this in the
 future though, if the possibility to keep a legacy version around is
 judged important enough.

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Ticket URL: <https://trac.macports.org/ticket/50703>
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