configure's tar ustar test can fail on Solaris

[Nelson H. F. Beebe]

David Shaw comments today:

>> ...
>> The thing is, the 'ustar' string is always at the same byte offset in
>> the file.  I'm wondering if "head -c 262 | tail -c 5" would do it, or
>> some sed or awk magic...
>> ...

Neither sed nor awk can handle binary files; gawk can, but may not be
universally available.

The tool you want is venerable dd: this works back to at least Unix
version V7:

	$ dd skip=257 bs=1 count=5 if=foo.tar 2>/dev/null | grep ustar
	ustar

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