Defines the type struct option and declares the variables
optarg, optind, opterr, optopt
and the functions getopt, getopt_long, getopt_long_only.
Documentation:
Portability problems fixed by Gnulib:
getopt_long is missing on some platforms:
Solaris 9.
getopt_long_only is missing on some platforms:
FreeBSD 5.2.1, NetBSD 10.0, Solaris 9, mingw.
getopt does not handle a leading ‘+’ character in
the options string on some platforms:
Solaris 11 2010-11.
Portability problems not fixed by Gnulib: