configure: gettext needs format macros too

The PRI macros are used in some of the translated strings, so libintl
needs to support them.

Note that need-formatstring-macros nonobviously implies need-ngettext.

configure.ac

@@ -453,7 +453,7 @@ dnl
 dnl Gettext stuff
 dnl
 AM_GNU_GETTEXT_VERSION([0.19.6])
-AM_GNU_GETTEXT([external], [need-ngettext])
+AM_GNU_GETTEXT([external], [need-formatstring-macros])
 
 dnl
 dnl Iconv stuff