Clarify the warning about the relative dates of Setup.dist and Setup;

Jeremy had seen the warning but not realized what he should do about
it.  Add the hint "Usually, copying Setup.dist to Setup will work."

Makefile.pre.in

@@ -376,6 +376,7 @@ Modules/Setup: $(srcdir)/Modules/Setup.dist
 		echo "Modules/Setup.dist is newer than Modules/Setup;"; \
 		echo "check to make sure you have all the updates you"; \
 		echo "need in your Modules/Setup file."; \
+		echo "Usually, copying Setup.dist to Setup will work."; \
 		echo "-----------------------------------------------"; \
 	fi