Fix build errors with -DNDEBUG.

        [BZ #18755]
        * iconv/skeleton.c (FUNCTION_NAME): Suppress -Wunused-but-set-variable
        warnings.
        * sysdeps/nptl/gai_misc.h (__gai_start_notify_thread): Same.
        (__gai_create_helper_thread): Same.
        * nscd/nscd.c (do_exit): Suppress -Wunused-variable.
        * iconvdata/iso-2022-cn-ext.c (BODY): Initialize local variable
        to suppress -Wmaybe-uninitialized warnings.