Change the mmap() feature test to check only for existence.

The autoconf-provided check used previously fails when the
platform doesn't support MAP_FIXED.  Apache doesn't use that
mmap() feature, so we don't want to require it.
Submitted by:	Greg Ames


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85457 13f79535-47bb-0310-9956-ffa450edef68