maint: relax autoconf requirement to v2.68.
maint: relax autoconf requirement to v2.68.

* configure.ac (AC_INIT): 2.68 is new enough, and is the newest
version available by default on Travis CI server.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

GUI/lyaml,GUI/lyaml