partially reverts 182f5a0f in the sense
that configure still insists on autoconf 2.68, but in a way that allows
to specify an already installed copy (that the libnumbertext build
already picked up successfully)

While there is autoconf268 package, it gets installed as autoconf268, but
aclocal doesn't provide a way to use something else than "autoconf"

In the spirit of "keeping it simple", no conditional check is done
whether libnumbertext is actually enabled or not.

Change-Id: Ice05a70ef56a4ed3428c74d15d6aeeaa54f71c0b
Reviewed-on: https://gerrit.libreoffice.org/67159Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
Tested-by: Jenkins