-This variable is missing on some platforms:
-MacOS X 10.3, FreeBSD 6.0, NetBSD 3.0, OpenBSD 3.8, IRIX 6.5, OSF/1 5.1, Cygwin.
+POSIX does not require this variable to be declared, and it is indeed not
+declared on some platforms:
+MacOS X 10.5, FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, IRIX 6.5, Solaris 11 2010-11.