diff --git a/tool/mkconfig.rb b/tool/mkconfig.rb
--- tool/mkconfig.rb
+++ tool/mkconfig.rb
@@ -123,10 +123,8 @@
     when /^arch$/
       if universal
         platform = val.sub(/universal/, %q[#{arch && universal[/(?:\A|\s)#{Regexp.quote(arch)}=(\S+)/, 1] || RUBY_PLATFORM[/\A[^-]*/]}])
       end
-    when /^includedir$/
-      val = '"$(SDKROOT)"'+val if /darwin/ =~ arch
     end
     v = "  CONFIG[\"#{name}\"] #{eq} #{val}\n"
     if fast[name]
       v_fast << v