*: add modern gettext

This patch updates gettext.m4 and related .m4 files and adds
gettext-runtime as a gmp/mpfr/... style host library, allowing newer
libintl to be used.

This patch /does not/ add build-time tools required for
internationalizing (msgfmt et al), instead, it just updates the runtime
library.  The result should be a distribution that acts exactly the same
when a copy of gettext is present, and disables internationalization
otherwise.

There should be no changes in behavior when gettext is included in-tree.
When gettext is not included in tree, nor available on the system, the
programs will be built without localization.

ChangeLog:

	PR bootstrap/12596
	* .gitignore: Add '/gettext*'.
	* configure.ac (host_libs): Replace intl with gettext.
	(hbaseargs, bbaseargs, baseargs): Split baseargs into
	{h,b}baseargs.
	(skip_barg): New flag.  Skips appending current flag to
	bbaseargs.
	<library exemptions>: Exempt --with-libintl-{type,prefix} from
	target and build machine argument passing.
	* configure: Regenerate.
	* Makefile.def (host_modules): Replace intl module with gettext
	module.
	(configure-ld): Depend on configure-gettext.
	* Makefile.in: Regenerate.

config/ChangeLog:

	* intlmacosx.m4: Import from gettext-0.22 (serial 8).
	* gettext.m4: Sync with gettext-0.22 (serial 77).
	* gettext-sister.m4 (ZW_GNU_GETTEXT_SISTER_DIR): Load gettext's
	uninstalled-config.sh, or call AM_GNU_GETTEXT if missing.
	* iconv.m4: Sync with gettext-0.22 (serial 26).

contrib/ChangeLog:

	* prerequisites.sha512: Add gettext.
	* prerequisites.md5: Add gettext.
	* download_prerequisites: Add gettext.

gcc/ChangeLog:

	* configure: Regenerate.
	* aclocal.m4: Regenerate.
	* Makefile.in (LIBDEPS): Remove (potential) ./ prefix from
	LIBINTL_DEP.
	* doc/install.texi: Document new (notable) flags added by the
	optional gettext tree and by AM_GNU_GETTEXT.  Document libintl/libc
	with gettext dependency.

libcpp/ChangeLog:

	* configure: Regenerate.
	* aclocal.m4: Regenerate.

libstdc++-v3/ChangeLog:

	* configure: Regenerate.

contrib/download_prerequisites

@@ -31,6 +31,7 @@ gmp='gmp-6.2.1.tar.bz2'
 mpfr='mpfr-4.1.0.tar.bz2'
 mpc='mpc-1.2.1.tar.gz'
 isl='isl-0.24.tar.bz2'
+gettext='gettext-0.22.tar.gz'
 
 base_url='http://gcc.gnu.org/pub/gcc/infrastructure/'
 
@@ -38,6 +39,7 @@ echo_archives() {
     echo "${gmp}"
     echo "${mpfr}"
     echo "${mpc}"
+    echo "${gettext}"
     if [ ${graphite} -gt 0 ]; then echo "${isl}"; fi
 }