lttng-babeltrace: add debug info support

Since Babeltrace 1.4, there is support for printing debug information
along with compatible traces and event records. Babeltrace needs
elfutils in this case.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
[Thomas:
 - add better description for
   0002-m4-ax_lib_elfutils.m4-add-cache-variable.patch
 - use automatic dependency on elfutils rather than a very complicated
   Config.in symbol]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>