are using our own ports of getopt or getopt_long, those will define
the variable for themselves; and if not, we don't need these, because
we never touch the variable anyway.
getopt_long(). This is more or less the same problem as we saw earlier
with getaddrinfo() and struct addrinfo, and for the same reason: random
user-added libraries might contain the subroutine, but there's no
guarantee we will find the matching header files.
