mkvtoolnix: fix Boost build, update to 8.4.0, allow Qt5 GUI

mkvtoolnix does not compile with newest Boost 1.58, mostly
due to the following bugs :
https://github.com/mbunkus/mkvtoolnix/issues/1172
https://github.com/mbunkus/mkvtoolnix/issues/1176



Update to the latest version, allow to use Qt5 if present,
and adapt configure steps accordingly.

Signed-off-by: Manuel Bachmann <manuel.bachmann@iot.bzh>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>