bug#20713: aclocal/tar.m4 and solaris 5.10

[Karl Berry]

Running a configure script on solaris 5.10 generated with automake-1.15,
I got these errors about id, which being pre-POSIX, does not support -u:

      id: illegal option -- u
      Usage: id [-ap] [user]
      id: illegal option -- g
      Usage: id [-ap] [user]
      checking whether UID 'unknown' is supported by ustar format... 
./configure: line 4190: test: unknown: integer expression expected
      no
      checking whether GID 'unknown' is supported by ustar format... 
./configure: line 4200: test: unknown: integer expression expected
      no

It comes from aclocal-1.15/tar.m4, which says:
      ... Errors in the 'id' calls
      # below are definitely unexpected, so allow the users to see them

Well, they are not "unexpected" on this system, so it's not especially
helpful to see them.  Still, I suppose it is not worth worrying about.
And I'm sure it is not worth parsing the output of this old id (with no
args passed) to get the info, but for the record, that would be:
uid=1053(karl) gid=1053(karl)

But what I do think would make sense is to avoid the subsequent error of
using a numeric test expression on a known non-numeric (the string
"unknown") when id fails.  After all, that much is tar.m4's own doing.
Perhaps simply change
  if test $am_uid -le $am_max_uid; then
to
  if test $am_uid != unknown && test $am_uid -le $am_max_uid; then
(and ditto gid).

Clearly nothing critical.  Just reporting ...

Best,
Karl