`__cplusplus` not set when `DUI::std` is empty/invalid

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The __cplusplus macro is not set when the provide DUI::std is either empty or invalid. This might lead to unexpected behavior.

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• if the provided standard is invalid we should bail out
• if no standard is being provided we should default to the latest

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• if the provided standard is invalid we should bail out

With #312 merged we do not bail out but provide an error via OutputList to indicate that the given standard is unknown.