Fix deprecated feature: target 'Dejagnu.swf' overrides 'Dejagnu.swf$(EXEEXT)'

Reported as deprecated by automake (GNU automake) 1.13.3

Indeed Dejagnu.swf is not technically a "program", so doesn't belong
in the check_PROGRAMS set
Add support for giflib 5.1
Add NEWS item about giflib-5.1 support (#42574)
Don't reallocate the audio buffer should it need to be resampled.

Also simplify things considerably.
Drop code from unsupported FFMPEG versions.
Pad, rather than double, the encoded buffers.
Drop code supporting unsupported FFMPEG versions.
'frame' does not own frame->data in this case.
Handle avcodec_decode_video2() return values. Clean up a bit.
Fix spacing.
Set the mime_type field for new libavformat (and hope there was no ffmpeg release between the addition of the field and the bumping of the ffmpeg version.)

Should fix the memory error detected by asan on the buildbots.
Fix typo.
Revert "Don't reallocate the audio buffer should it need to be resampled."

This reverts commit bebc26da7dcbbae45280f609ffce34a612b02cd4.

This change needs a bit more work.
Merge remote-tracking branch 'gnu/master'

Conflicts:
	libbase/GnashImageGif.cpp