configure.ac: change default minimal log level from VERBOSE to DEBUG

The goal here is that Babeltrace users have enough logging by default
without impacting the performance too much. From what I measured once
this patch is applied, the performance is very similar to an INFO build.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I20346f0f3eecf725e730a3e5add94804637c7193
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1445
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>