Honor $(DESTDIR) when installing sample files

samples/Makefile.am

@@ -15,10 +15,10 @@ knot.sample.conf: Makefile
 knot.sample.conf: knot.sample.conf.in
 
 install-data-local: knot.sample.conf
-	[ -d $(sysconfdir) ] || \
-	  $(INSTALL) -d $(sysconfdir)
-	[ -f $(sysconfdir)/knot.sample.conf ] || \
-	  $(INSTALL_DATA) knot.sample.conf example.com.zone $(sysconfdir)
+	[ -d $(DESTDIR)/$(sysconfdir) ] || \
+	  $(INSTALL) -d $(DESTDIR)/$(sysconfdir)
+	[ -f $(DESTDIR)/$(sysconfdir)/knot.sample.conf ] || \
+	  $(INSTALL_DATA) knot.sample.conf example.com.zone $(DESTDIR)/$(sysconfdir)
 
 clean-local:
 	rm -f knot.sample.conf