configure: require autoconf 2.68

LO's own configure doesn't need it, but the
ExternalProject_libnumbertext requires this version; since it's
available in RHEL6 as "autoconf268", just keep it simple and require it
in the top-level configure.

Change-Id: I43a6ef10089363c344f06134d75f54685ed7026b
Reviewed-on: https://gerrit.libreoffice.org/67002
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

configure.ac

@@ -11,7 +11,7 @@ dnl in order to create a configure script.
 
 AC_INIT([LibreOffice],[6.3.0.0.alpha0+],[],[],[http://documentfoundation.org/])
 
-AC_PREREQ([2.59])
+AC_PREREQ([2.68])
 
 if test -n "$BUILD_TYPE"; then
     AC_MSG_ERROR([You have sourced config_host.mk in this shell.  This may lead to trouble, please run in a fresh (login) shell.])