autoreconf: Support AM_GNU_GETTEXT_REQUIRE_VERSION

autoreconf: Support AM_GNU_GETTEXT_REQUIRE_VERSION

As reported in https://github.com/gentoo/gentoo/pull/163, gettext 0.19.6
supports using AM_GNU_GETTEXT_REQUIRE_VERSION (specifying a *minimum*
required version of gettext) instead of AM_GNU_GETTEXT_VERSION
(which specifies a *fixed* required version of gettext).
Update autoreconf to support both.

 * bin/autoreconf.in (autoreconf_current_directory): Check for
   AM_GNU_GETTEXT_REQUIRE_VERSION as well as AM_GNU_GETTEXT_VERSION.
   Update diagnostics about using AM_GNU_GETTEXT_VERSION but not
   AM_GNU_GETTEXT, or vice versa, to match.
 * doc/autoconf.texi (autoreconf Invocation): Update to match.