common: support custom, extra information for build's version

common: support custom, extra information for build's version

This patch adds a system of extra version information also found in
LTTng-tools.

`src/common/Makefile` generates `src/common/version.i` at every build.
This file contains:

* The current Git revision description.

* Extra name of the version (found in `version/extra_version_name`).

* Extra description of the version (found in
  `version/extra_version_description`).

* A list of patch file names found in `version/extra_patches`.

All definitions can be empty strings.

See `version/README.adoc` to learn more.

As of this patch, libbabeltrace2 does not offer getters for this data
and the CLI does not print it with the `--version` option. This is
reserved for subsequent patches.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ife50e5bcaa6b3bdeda6ee4e7c1fdeb2fb1f63887
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2836
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>