Fix r33904.

It breaks following tests if RUBYOPT=-w
* test_class_build(TestGemExtExtConfBuilder)
* test_class_build_rbconfig_make_prog(TestGemExtExtConfBuilder)
* test_class_build_env_make(TestGemExtExtConfBuilder)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@33905 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -53,6 +53,8 @@ module MakeMakefile
   $static = nil
   $config_h = '$(arch_hdrdir)/ruby/config.h'
   $default_static = $static
+  $OBJEXT = nil
+  $ignore_error = ''
 
   unless defined? $configure_args
     $configure_args = {}