Move SVN_UNUSED into a private header.

* subversion/include/svn_types.h
  (SVN_UNUSED): Remove and move to ...

* configure.ac
  (AH_BOTTOM): to here ...

* subversion/svn_private_config.hw
  (SVN_UNUSED): and here.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1616257 13f79535-47bb-0310-9956-ffa450edef68
Use SVN_UNUSED in some places where already casting to void.

* subversion/bindings/wig/python/libsvn_swig_py/swiguilt.py.c
  (make_ob_pool): Use SVN_UNUSED.

* subversion/libsvn_fs/editor.c
  (UNUSED): Remove macro.
  (alter_symlink_cb): Use SVN_UNUSED instead of UNUSED.

* subversion/libsvn_subr/cache-memcache.c
  (memcache_is_cachable): Use SVN_UNUSED.

* subversion/libsvn_subr/utf8proc.c
  (UNUSED): Remove macro.
  (svn_utf__utf8proc_version): Use SVN_UNUSED instead of UNUSED.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1616267 13f79535-47bb-0310-9956-ffa450edef68