ap_exists_config_define() function is now "public"

PR:
Obtained from:
Submitted by:	Doug MacEachern
Reviewed by:	Jim, Ben Hyde


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@82413 13f79535-47bb-0310-9956-ffa450edef68