Refs #37833 - Fix file timestamp handling when pulling translations

The Ruby gettext library wants the .po.time_stamp file to be at least as
new as the .po file. If it doesn't exist or is older, it will remove the
.edit.po file and then copy the .po file to .edit.po. In practice the
Makefile doesn't guarantee this, causing translations to be ignored.

A more concrete example what ends up happening:

    make -C locale tx-pull # updates $domain.edit.po

    # This all assumes $domain.po exists
    if [ ! -f $domain.po.time_stamp ] -o [ $domain.po -nt $domain.po.time_stamp ] ; then
        rm -f $domain.edit.po
    fi
    if [ ! -f $domain.edit.po ] ; then
        cp $domain.po $domain.edit.po

        # Merge in the new messages from the template
        msgmerge --update --sort-by-file --no-wrap $domain.edit.po $domain.pot
        if [ -f $domain.po ] -a [ $domain.po -nt $domain.edit.po ] ; then
            msgmerge --output $domain.edit.po --sort-by-file --no-wrap --no-obsolete-entries $domain.po $domain.edit.po
        fi
    fi

This ensures that $domain.po.time_stamp is always the same mtime as
$domain.po which prevents the gettext library from removing the updated
$domain.edit.po file.

It means there is no message merging and we rely on Transifex for doing
that part for us.

locale/Makefile

@@ -31,6 +31,16 @@ all-mo: $(MOFILES)
 
 check: $(POXFILES)
 
+%.edit.po: %.po.time_stamp
+	touch $@
+
+# gettext will trash the .edit.po file if the time stamp doesn't exist or is older than the po file
+%.po.time_stamp: %.po
+	touch --reference $< $@
+
+# Prevent make from treating this as an intermediate file to be cleaned up
+.PRECIOUS: %.po.time_stamp
+
 # Unify duplicate translations
 uniq-po:
 	for f in $(shell find ./ -name "*.po") ; do \