* 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


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@535169 13f79535-47bb-0310-9956-ffa450edef68