Re-addition of __getopt_argv_const caused

Re-addition of __getopt_argv_const caused
redefinition warnings. To avoid them, include the defines
in `#if !defined __need_getopt ... #endif'. The only place
where __getopt_argv_const is used is in definitions
of getopt_long and getopt_long_only below, which are as well
protected by `#ifndef __need_getopt'.
[defined __GETOPT_PREFIX && !defined __need_getopt]: Undef
__need_getopt after including <stdio.h> and <unistd.h> These
headers might have defined it.