gnu: autotools: Add Autoconf 2.68; turn 'autoconf-wrapper' into a procedure.

* gnu/packages/autotools.scm (autoconf-2.68): New variable.
  (autoconf-wrapper): Turn into a procedure.  Turn comment into a
  docstring.
  (automake): Adjust accordingly.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>