Require ngettext in test of system gettext implementation

gcc currently uses ngettext in a number of places (gcc/cp/pt.c,
gcc/diagnostic.c, gcc/collect2.c).  Apparently there are (or used to
be) gettext implementations that lack ngettext.  See config/gettext.m4.

This patch arranges for intl/ to be compiled when the system gettext
lacks ngettext.

	* configure.ac: Invoke AM_GNU_GETTEXT with need_ngettext.
	* configure: Regenerate.

From-SVN: r254486