extconf.h dependency

* lib/mkmf.rb (MakeMakefile#depend_rules): deal with extconf.h
  dependency.  just remove it and bail out right now.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@35510 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -1897,7 +1897,14 @@ RULES
     unless suffixes.empty?
       depout.unshift(".SUFFIXES: ." + suffixes.uniq.join(" .") + "\n\n")
     end
-    depout.unshift("$(OBJS): $(RUBY_EXTCONF_H)\n\n") if $extconf_h
+    if $extconf_h
+      depout.unshift("$(OBJS): $(RUBY_EXTCONF_H)\n\n",
+                     "$(RUBY_EXTCONF_H): $(srcdir)/extconf.rb\n",
+                     "$(RUBY_EXTCONF_H):\n",
+                     "\t$(Q)$(RM) $(@)\n",
+                     "\t$(ECHO) $@ is outdated, re-configure\n",
+                     "\t@exit 1\n")
+    end
     depout.flatten!
     depout
   end