* bootstrap: Set XGETTEXT_OPTIONS to flag our printf-format functions,

* bootstrap: Set XGETTEXT_OPTIONS to flag our printf-format functions,
so that translators are warned about strings that are formats but
don't look like formats.  This might help prevent core dumps.
* configure.ac (AM_GNU_GETTEXT): Upgrade to need-formatstring-macros.
Suggested by Eric Blake to avoid problems like
<http://lists.gnu.org/archive/html/bug-coreutils/2006-07/msg00087.html>.
(AM_GNU_GETTEXT_VERSION): Upgrade from 0.12.1 to 0.15.  The gettext
manual says we should use the version number normally expected of
maintainers, neither more nor less.