* Allow suppression of including <optional> via macro
* Combine preprocessor conditions
* Make inclusion of header <optional> opt-in
* Make inclusion of <optional> opt-out via macro
* Auto-detect C++17+ when defining macro
* Enclose macro expression in parentheses
* Check value of macro, not whether it's defined
* Fix parentheses
* Don't define macro with expression using other macros
The preprocessor doesn't seem to like it.
* Fix parentheses
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