build: Use a minimum gettext requirement rather than an exact one

In configure.ac, specify the minimum gettext version we require, rather
than the exact one. This fixes a situation where the autoconf macros
used for gettext will be the latest available on the system (for
example, 0.19); but the copied-in Makefile.in.in will be for the exact
version specified in configure.ac (in this case, 0.18).

In that situation, the gettext build rules will error out at `make` time
with the message:
   *** error: gettext infrastructure mismatch: using a Makefile.in.in
   from gettext version 0.18 but the autoconf macros are from gettext
   version 0.19

Avoid that by specifying a minimum version dependency rather than an
exact one. This should not cause problems as we haven’t committed any
generated or external gettext files into git, so each developer will end
up regenerating the build system for their system’s version of gettext,
as expected.

See the subsection of
https://www.gnu.org/software/gettext/manual/html_node/Version-Control-Issues.html
for more information.

Note that autoreconf currently doesn’t recognise
AM_GNU_GETTEXT_REQUIRE_VERSION, so we must continue also using
AM_GNU_GETTEXT_VERSION. autopoint will ignore the latter if the former
is present. See
https://lists.gnu.org/archive/html/autoconf-patches/2015-10/msg00000.html.

This bumps our gettext dependency to 0.19.6, which is when
AM_GNU_GETTEXT_REQUIRE_VERSION was introduced.

https://bugzilla.gnome.org/show_bug.cgi?id=777430