Adopt conditional handling for the new AC_USE_SYSTEM_EXTENSIONS as of 2.60.

Submitted by: Arfrever Frehtes Taifersar Arahesis <arfrever.fta gmail.com>
Backports: r670003


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@670004 13f79535-47bb-0310-9956-ffa450edef68