Systems with non-posix TAR implementations: How to handle

Ref. the testlog that I posted in the thread "[PATCH] Add Tru64/OSF1
support in Makefile"

*** t0024-crlf-archive.sh ***
*   ok 1: setup
* FAIL 2: tar archive


               git archive --format=tar HEAD |
               ( mkdir untarred && cd untarred && "$TAR" -xf - )

               test_cmp sample untarred/sample


 *   ok 3: zip archive.

The problem with that test is that the Tru64 V5.1 'tar' says 'This
doesn't look like a tar archive' on the generated file. However, it
turns out that the tar file generated by 'git archive --format=tar' is
fine, it's just that it's a POSIX tar archive and the Tru64 'tar'
program isn't POSIX tar format compatible.
How should we handle this? Just ignore errors in this test on Tru64,
or is there some other way that I missed?

-Tor