The library mis-match on ubuntu 18.04 seems impossible to fix, so
revert to using the system binaries.
This makes including a subtitle stream impossible for now, but moving
to the concat *filter* had made that pretty much impossible anyway,
because ffmpeg (even in HEAD) does not set the right PTS when the
input is in matroska format.
The release upgrade from 16.04 to 18.04 was not seamless.
LD_LIBRARY_PATH= ld ffmpeg.git/ffmpeg
ld: warning: libcdio_paranoia.so.1, needed by //usr/lib/x86_64-linux-gnu/libavdevice-ffmpeg.so.58, not found (try using -rpath or -rpath-link)
ld: warning: libcdio_cdda.so.1, needed by //usr/lib/x86_64-linux-gnu/libavdevice-ffmpeg.so.58, not found (try using -rpath or -rpath-link)
ld: warning: libbluray.so.1, needed by //usr/lib/x86_64-linux-gnu/libavformat-ffmpeg.so.58, not found (try using -rpath or -rpath-link)
ld: warning: libx265.so.79, needed by //usr/lib/x86_64-linux-gnu/libavcodec-ffmpeg.so.58, not found (try using -rpath or -rpath-link)
