configure.ac: Enable 'tar-pax' by default

configure.ac: Enable 'tar-pax' by default

Automake defaults to the 'v7' legacy tar format in GNU tar, through
passing of the '-o' parameter to GNU tar. Enabling this option results
in errors for users with 32 bit UIDs:

    $ make dist
    ...
    tardir=openvswitch-2.3.90 && ${TAR-tar} chof - "$tardir" | GZIP=--best gzip -c >openvswitch-2.3.90.tar.gz
    tar: value 12345678 out of uid_t range 0..2097151
    tar: Exiting with failure status due to previous errors
    make[1]: Leaving directory `/development/ovs'
    ...

The 'tar-pax' format is a 2001 POSIX standard that allow longer file
names and other niceties. Its use is a configuration option in Automake
1.9+. Enable this option.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Mark D. Gray <mark.d.gray@intel.com>
Cc: Panu Matilainen <pmatilai@laiskiainen.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>