APR keeps some symbols that we need hidden away in a private header.

* subversion/include/private/svn_dep_compat.h
  (APR_FREADONLY, APR_OPENINFO): Conditionally define constants.
* subversion/libsvn_subr/io.c: Include svn_dep_compat.h.
  (APR_FREADONLY): Remove this definition.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1534981 13f79535-47bb-0310-9956-ffa450edef68