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While this is more useful for libraries, some of our plugins with multiple
files and some internal API can also benefit from this.

Libav-incompatible changes were introduced to support
FFmpeg and we can no longer properly support Libav.

FFmpeg micro versions for libav* start at 100 (this was
done to differentiate from Libav builds). We use this
to bail at configure time if the system libav* libraries
are not provided by FFmpeg.

https://bugzilla.gnome.org/show_bug.cgi?id=758183

https://bugzilla.gnome.org/show_bug.cgi?id=756870

This reverts commit ac343715.

Doesn't actually make sense as it will put the (uninstalled) library paths
into the installed .la files. How does this all work?

It's needed only for subtitle charset conversion, and we don't
use the ffmpeg subtitle support anyway.

Also disable d3d11va and dxva2 support, we don't use the hardware
codec support.

VideoDecodeAcceleration framework is deprecated in 10.11, and currently
cuases linker errors when compiling on OSX. Oddly, --disable-hwaccels
did not also disable h264_vda already.

https://bugzilla.gnome.org/show_bug.cgi?id=751838

ffmpeg seems to be the one of the two forks, which is most widely used by
Linux distributions and in general. Also Google is using it for e.g. Chrome
and has engineers working on finding and fixing security issues in it.

https://bugzilla.gnome.org/show_bug.cgi?id=751607

It enables GStreamer to decode Chinese AVS video streams.
Also fix indentation.

https://bugzilla.gnome.org/show_bug.cgi?id=727731

This is only required for builds from git, people can still
build tarballs if they only have older autotools.

https://bugzilla.gnome.org//show_bug.cgi?id=747624

They are still bad, but if we don't want to have them, we should rather remove this totally.

Nowadays libav has proper releases and API/ABI guarantees.
We're not living in last decade.