5 POSIX specification:@* @url{https://pubs.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 Mac OS X 10.5, IRIX 6.5, MSVC 14.