Add support for functions returning VOID (i.e. non-standard extension, allowing functions that do not return any data)

Add -warn flag when running flex -> warn of potential bugs

Add support for non-standard VOID datatype. May ony be used to define functions returning VOID.

Delete the YY_NO_UNPUT define, as we do use the unput() function.

Add support for non standard feature: allow POUs with no in, out and inout parameters

Add option to not add the EN and ENO parameters to every function and FB (generated C code becomes smaller, and uses less memory for function invocations)

Correctly identify errors when parsing erroneous code (make sure flex goes back to INITIAL state when code contains errors that do not allow determining whether ST or IL is being parsed)