[1/2] include/wine: add option to specify the unittest target via environment variable

[Juan Lang]

>> To get around this problem and make a valid unittest without fixing the
>> problem, i looked for another way to pass the test target to the
>> executable.
>
> This statement classifies this patch as a hack, not?

Perhaps, but there's a chicken and egg problem:  fixing the problem
without a valid unit test doesn't seem likely to happen.
--Juan