Create a preprocessor conditional

Add a simple macro definition from a string (e.g., for a `-D` option passed on the command line

Called when we reach the end of an input stream. Performs some validation and then destroys the input stream if required.

Evaluate one infix operation in a preprocessor conditional expression

Look up the directive with the given name.

Process a directive, where the preprocessor has already consumed the `#` token that started the directive line.