Makefile.in: no probes.stamp unless DTRACE_REBUILD

* Makefile.in (probes.o): do not need to rebuild object files
  unless `dtrace -G` modifies input object files.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49956 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Makefile.in

@@ -393,7 +393,7 @@ probes.stamp: $(DTRACE_REBUILD_OBJS)
 	fi
 	$(Q) touch $@
 
-probes.@OBJEXT@: $(srcdir)/probes.d probes.stamp
+probes.@OBJEXT@: $(srcdir)/probes.d $(DTRACE_REBUILD:yes=probes.stamp)
 	@$(ECHO) processing probes in object files
 	$(Q) $(RM) $@
 	$(Q) $(DTRACE) -G -C $(INCFLAGS) -s $(srcdir)/probes.d -o $@ $(DTRACE_REBUILD_OBJS)