Permit building with apr-2 (aka trunk).

* build/ac-macros/apr.m4
  (SVN_LIB_APR): Add major 2 to the list of acceptable majors.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1560586 13f79535-47bb-0310-9956-ffa450edef68