configure.ac, lib: rename "extra" (version) to "development stage"

configure.ac, lib: rename "extra" (version) to "development stage"

"Extra" is a term which we'll use for something else brought by a
subsequent patch.

I took the "development stage" term from
<https://en.wikipedia.org/wiki/Software_release_life_cycle#Stages_of_development>,
where "Release candidate" is one of the stages.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I285fbf9851cde41a520079b4c31cdc5d8bf32412
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2835
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>