mkmf.rb: defer use of instance variable

* lib/mkmf.rb (MakeMakefile#dir_config, MakeMakefile#_libdir_basename):
  defer use of instance variable until needed.  [Bug #8074]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39717 b2dd03c8-39d4-4d8f-98ff-823fe69b080e