[krbdev.mit.edu #8887] krb 1.18 build failure on AIX 6.1

[Greg Hudson via RT]

<URL: https://krbdev.mit.edu/rt/Ticket/Display.html?id=8887 >

O_NOFOLLOW is in POSIX.1-2008. AIX 6.1 (released in 2007) reached its end of
support life in 2017. I don't think a change is required.