In 094a8c82 we added the ability to compile a source file relative to
_STARTDIR$ instead of the location of the qb64pe executable. This change
fixes `-o` so that it is also treated as relative to _STARTDIR$ when the
provided source file is.
Note that this is not a real behavior change because if the provided
source file is relative to the qb64pe executable then `-o` will be as
well, which matches the previous behavior. The cases where it is made
relative to _STARTDIR$ all produced errors until 094a8c82 was done, so
there was no previous defined behavior.
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