fix: obsolete warnings with autoconf >= 2.71

fix: obsolete warnings with autoconf >= 2.71

Disable 'obsolete' warnings when running autoreconf, on systems with
autoconf >= 2.71 the glib2 macros which we have no control over will
trigger warnings, which then confuses jenkins's compiler warnings
detection code and fails the jobs.

Change-Id: I000e1ac4badcaaed8035f2765ad2e128e7433b63
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>