Merge r535169 from trunk:

* Add extern "C" linkage to several headers to make it easier to use
  them in C++ code.

PR: 42286
Submitted by: Davi Arnaut <davi haxent.com.br>
Reviewed by: rpluem, wrowe, niq


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@536627 13f79535-47bb-0310-9956-ffa450edef68