5 POSIX specification: @url{http://www.opengroup.org/onlinepubs/9699919799/functions/optarg.html}
9 Portability problems fixed by Gnulib:
13 Portability problems not fixed by Gnulib:
16 This variable is missing on some platforms:
17 MacOS X 10.3, IRIX 6.5, OSF/1 5.1, Cygwin 1.5.x.