gnu: Add ffmpeg-jami.

gnu: Add ffmpeg-jami.

This package is needed because Jami uses a modified version
of ffmpeg, which provides GPU hardware acceleration, automatical
adapting of bitrate and extra codecs. Because of the configure flags
list being long, it is better to keep them separated in variables,
instead of littering the package definition.

* gnu/packages/jami.scm (ffmpeg-jami, %ffmpeg-default-configure-flags,
%ffmpeg-linux-configure-flags, %ffmpeg-linux-x86-configure-flags):
New variables.
(ffmpeg-compose-configure-flags): New procedure.
(libring)[inputs]: Use ffmpeg-jami instead of ffmpeg.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>