We should define SVN_HAS_DUNDER_BUILTINS under Windows as well
(as "optimization not supported")

* subversion/svn_private_config.hw
  (SVN_HAS_DUNDER_BUILTINS): define as "not supported"

Suggested by: danielsh

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1508460 13f79535-47bb-0310-9956-ffa450edef68