Be more explicit about when IRIXNIS=yes is required. On recent

systems, libsun is empty and on some it has been rumored to cause
problems if you link against it.

Inspired by PR: 2050


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80856 13f79535-47bb-0310-9956-ffa450edef68