mkconfig.rb: fix anchor

* tool/mkconfig.rb: should match whole word.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39100 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/mkconfig.rb

@@ -78,7 +78,7 @@ File.foreach "config.status" do |line|
     when /^RUBY_INSTALL_NAME$/; next if $install_name
     when /^RUBY_SO_NAME$/; next if $so_name
     when /^arch$/; if val.empty? then val = arch else arch = val end
-    when /^sitearch/; val = '$(arch)' if val.empty?
+    when /^sitearch$/; val = '$(arch)' if val.empty?
     end
     case val
     when /^\$\(ac_\w+\)$/; next