* libc/include/getopt.h: Hide newlib extensions under
the __need_getopt_newlib flag.
* libc/stdlib/getopt.c: #define __need_getopt_newlib before including
getopt.h. Put entire code under !HAVE_GETOPT to support platforms
with their own getopt implementation.
* configure.host: Specify HAVE_GETOPT for x86-linux.