Default to use -flat_namespace on Darwin expect for v1.3 and prior (including rhapsody).

Submitted by:	Sander Temme
Reviewed by:	Wilfredo Sanchez


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@92175 13f79535-47bb-0310-9956-ffa450edef68