build: lower gettext requirements.

build: lower gettext requirements.

Bison was uselessly requiring the formatstring macros from
gettext, which resulted in mo files not being installed on systems
that perfectly supported Bison mo files.  Lower the requirement.
http://lists.gnu.org/archive/html/bug-bison/2009-08/msg00006.html

	* configure.ac: Require need-ngettext instead of
	need-formatstring-macros.
	Reported by Martin Jabocs.
	Suggested by Bruno Haible.
	* INSTALL: Restructure.
	(Internationalization): New.
(cherry picked from commit b9e42bb439643b6b193ba44696c370bbb35d2ec0)