Support cross-testing (version 2)

[Roland McGrath]

> How about test-wrapper and test-wrapper-env (to run a newly built program, 
> to run a newly built program with environment variables)?  With the 
> default of test-wrapper-env being "$(test-wrapper) env".  The 
> run-built-tests default when cross-compiling would then depend on whether 
> $(test-wrapper) is empty or not.

That seems reasonable.  It's unfortunate that env does not grok --
as a separator between variable-setting arguments and the command.
That would let us make it a mandatory convention that -- appear in
$(test-wrapper-env) uses, making make it easy and robust to write
test-wrapper-env scripts.  But matching *=* with case is not so hard.


Thanks,
Roland