Merge r1340286:

    Make mkdir.sh save to use in parallel builds:
    Don't fail if a formerly missing directory has been created by another process
    in the meantime.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr-util/branches/1.5.x@1460349 13f79535-47bb-0310-9956-ffa450edef68