Enable autotools warnings as errors

Enable autotools warnings as errors

Enable all autotools warnings and error out to help catch all the
wonderful, shall we say, pecularities of M4sh.

  * Bump the minimum autoconf version to 2.69, we already require
    automake 1.12 released in 2012, use an autoconf of the same vintage.

  * Minor syntax tweaks by running 'autoupdate'.

  * Use AS_ESCAPE to escape bt_version_description C string

  * Rename obsolete AC_HELP_STRING to AS_HELP_STRING.

  * Use pkg-config to detect elfutils instead of our custom macro using
    AC_RUN_IFELSE which won't work when cross-compiling.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: Ic4de35dc6a51db3fd82b145e8a4ce4bb80e6b27b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3812
CI-Build: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>