[Bug-tar] Please fix the build with gcc 8.1

[Pavel Raiskup]

With current gnulib submodule hash - build system throws:

          CC       stdopen.o
        cc1: error: -Wabi won't warn about anything [-Werror=abi]
        cc1: error: -Wabi won't warn about anything [-Werror=abi]
        cc1: note: -Wabi warns about differences from the most up-to-date ABI, 
which is also used by default
        cc1: note: -Wabi warns about differences from the most up-to-date ABI, 
which is also used by default
        cc1: note: use e.g. -Wabi=11 to warn about changes from GCC 7
        cc1: note: use e.g. -Wabi=11 to warn about changes from GCC 7
        cc1: error: -Wabi won't warn about anything [-Werror=abi]
        cc1: note: -Wabi warns about differences from the most up-to-date ABI, 
which is also used by default
        cc1: note: use e.g. -Wabi=11 to warn about changes from GCC 7
        cc1: error: -Wabi won't warn about anything [-Werror=abi]
        cc1: note: -Wabi warns about differences from the most up-to-date ABI, 
which is also used by default
        cc1: note: use e.g. -Wabi=11 to warn about changes from GCC 7
        cc1: error: -Wabi won't warn about anything [-Werror=abi]
        cc1: note: -Wabi warns about differences from the most up-to-date ABI, 
which is also used by default
        cc1: note: use e.g. -Wabi=11 to warn about changes from GCC 7
        cc1: all warnings being treated as errors

With updated gnulib to the latest git version - the build system shows:

          CC       rtapelib.o rtapelib.c: In function ‘rmt_ioctl__’:
        rtapelib.c:675:18: error: cast increases required alignment of target 
type [-Werror=cast-align]
          uintmax_t u = (((struct mtop *) argument)->mt_count < 0
                          ^
        rtapelib.c:676:27: error: cast increases required alignment of target 
type [-Werror=cast-align]
                  ? - (uintmax_t) ((struct mtop *) argument)->mt_count
                                   ^
        rtapelib.c:677:27: error: cast increases required alignment of target 
type [-Werror=cast-align]
                  :   (uintmax_t) ((struct mtop *) argument)->mt_count);
                                   ^
        rtapelib.c:684:7: error: cast increases required alignment of target 
type [-Werror=cast-align]
          if (((struct mtop *) argument)->mt_count < 0)
               ^
        rtapelib.c:690:5: error: cast increases required alignment of target 
type [-Werror=cast-align]
            ((struct mtop *) argument)->mt_op, p);
             ^
        rtapelib.c:735:7: error: cast increases required alignment of target 
type [-Werror=cast-align]
          if (((struct mtget *) argument)->MTIO_CHECK_FIELD < 256)

Pavel