Recognize the current AIX 5L release (V5R1).

Apache builds and runs without this, but the expected
default mutex mechanism isn't used and the DSO implementation
is not as expected either.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@91367 13f79535-47bb-0310-9956-ffa450edef68