Use the newest gettext release.

* srcm4/po.m4: New file, from gettext-1.0.
* Makefile.devel (srclib/Makefile.gnulib): Don't override it with the
older version from gnulib.
* DEPENDENCIES: Mention that GNU make >= 4.0 is required.
* po/Makefile.in.in: Upgrade to gettext-1.0, keeping the change
	2019-01-27  Bruno Haible  <bruno@clisp.org>
	Accommodate a shell that is not in /bin/sh.
	* Makefile.in.in (SHELL): Use the value found by configure.
* po/Rules-quot: Upgrade to gettext-1.0.
* po/boldquot.sed: Upgrade to gettext-1.0.
* po/en@boldquot.header: Upgrade to gettext-1.0.
* po/en@quot.header: Upgrade to gettext-1.0.
* po/insert-header.sed: Upgrade to gettext-1.0.
* po/quot.sed: Upgrade to gettext-1.0.
* po/Makevars (PO_DEPENDS_ON_POT): Remove variable.
* po/fetch-po: New file.