Add DSO support for OpenStep (Mach 4.2) platform.

Submitted by: Rex Dieter <rdieter@math.unl.edu>
Ported to 1.3.6: Ralf S. Engelschall
PR: 3997


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@83216 13f79535-47bb-0310-9956-ffa450edef68