gettextize: Use a crafted version string
Reported by Max Lin in:
<https://lists.gnu.org/archive/html/bug-gettext/2015-07/msg00024.html>.
* gettextize.in: Replace AM_GNU_GETTEXT_VERSION invocation with
"$gettext_version" rather than "$version". This makes it easier
to maintain a minor release without being affected by the actual
version number generated by git-version-gen.
(gettext_version): New variable.
