Depending on the #include order of svn_private_config.h vs.
svn_hash.h, SVN_HAS_DUNDER_BUILTINS may not be defined when
the svn_hash_gets macro is being parsed.

Generate a compiler warning when svn_hash.h gets #included
before svn_private_config.h.

* subversion/include/svn_hash.h
  (SVN_HAS_DUNDER_BUILTINS): disable optimization when undefined
  (svn_hash_gets): extend commentary

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1508222 13f79535-47bb-0310-9956-ffa450edef68