Ticket #4466: fix sed usage in po/Makefile.

'po' directory does not use automake assignment.

This is a correction for f45d02ba2ad00175874c77a76481678498fd0abb.

Closes MidnightCommander/mc#191.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

po/Rules-pot-defaults

@@ -1,7 +1,7 @@
 
 update-po: Makefile
 	$(MAKE) $(DOMAIN).pot-update && \
-	$(SED) \
+	$${SED-sed} \
 	    -e '/#, fuzzy/d' \
 	    -e 's/Project-Id-Version: PACKAGE VERSION/Project-Id-Version: $(PACKAGE) $(VERSION)/' \
 	    -e 's/^"Plural-Forms: nplurals=INTEGER;/# "Plural-Forms: nplurals=INTEGER;/' \