intl/tst-gettext: fix failure with newest msgfmt

Since upstream gettext commit d13f165b83 (msgfmt: Remove
POT-Creation-Date field from the header in the output.), msgfmt does not
copy the POT-Creation-Date field in the header entry from the po file to
the mo file anymore. This breaks the assumption that we can test gettext
by comparing each message in the po files with the corresponding string
return by gettext. This makes the intl/tst-gettext to fail.

While it would have been possible to modify the po2test.awk script to
also strip the line POT-Creation-Date field when creating the msgs.h
file, it would not work with both the old and new msgfmt.

Instead create a tst-gettext-de.po file from de.po by removing the
POT-Creation-Date line. Another alternative would be to use a static
tst-gettext-de.po file, but I guess the reason for using de.po is to
also catch issues caused by newly added strings.

As tst-catgets also uses msg.h, it should also be updated. Instead of
using the new tst-gettext-de.po file, the patch modifies xopen-msg.awk
to avoid creating a second catgets->intl dependency.

Changelog:
	[BZ #21508]
	* catgets/xopen-msg.awk: Ignore POT-Creation-Date line.
	* intl/Makefile ($(objpfx)tst-gettext-de.po): Generate
	intl/tst-gettext-de.po from po/de.po by removing the
	POT-Creation-Date line.
	($(objpfx)msgs.h): Depend on $(objpfx)tst-gettext-de.po instead of
	../po/de.po.
	* intl/tst-gettext.sh: Use ${objpfx}tst-gettext-de.po instead of
	../po/de.po.
(cherry picked from commit 56456a2a)