Introduce SVN_EXPERIMENTAL as a counterpart to SVN_DEPRECATED.
it has less compiler support than the latter but at least GCC
issues a warning.

* subversion/include/svn_types.h
  (SVN_EXPERIMENTAL): define new function marker

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1526012 13f79535-47bb-0310-9956-ffa450edef68