diff --git a/ChangeLog b/ChangeLog
index 0107978ef0..630f7c2f3c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Oct 27 08:56:11 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* mkconfig.rb: reverted r25443 because build_os is used in mkmf.rb
+	  on some platforms.  [ruby-core:26332]
+
 Tue Oct 27 07:53:25 2009  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* gc.c (garbage_collect_with_gvl): do not garbage_collect when
diff --git a/tool/mkconfig.rb b/tool/mkconfig.rb
index 846b6ab880..ca1f46713a 100755
--- a/tool/mkconfig.rb
+++ b/tool/mkconfig.rb
@@ -77,7 +77,6 @@ File.foreach "config.status" do |line|
     when /^RUBY_SO_NAME$/; next if $so_name
     when /^arch$/; if val.empty? then val = arch else arch = val end
     when /^sitearch/; val = '$(arch)' if val.empty?
-    when /^build/; next
     end
     case val
     when /^\$\(ac_\w+\)$/; next