package/diffutils: disable getopt_long replacement with glibc

package/diffutils: disable getopt_long replacement with glibc

Since glibc >= 2.26, don't try to use getopt_long replacement bundled
with diffutils. It will conflict with the one from glibc.
The getopt copy bundled in glibc has been updated with the one from gnulib.

After a lot of warnings...
Fixes:
xstrtol-error.c: In function 'xstrtol_error':
xstrtol-error.c:84:26: error: invalid use of undefined type 'struct rpl_option'
     option = long_options[opt_idx].name;
                          ^
xstrtol-error.c:84:26: error: dereferencing pointer to incomplete type 'const struct rpl_option'

[...]

xstrtol-error.c:92:1: error: conflicting types for 'xstrtol_fatal'
 xstrtol_fatal (enum strtol_error err,
 ^~~~~~~~~~~~~
In file included from xstrtol-error.c:20:0:
xstrtol.h:69:16: note: previous declaration of 'xstrtol_fatal' was here
 _Noreturn void xstrtol_fatal (enum strtol_error,
                ^~~~~~~~~~~~~

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>