Provide some guidance about the order or #define and #include statements in cobj

[gprossliner]

When implementing the cobj files for a project, some code was not generated, which was expected to be so. The problem was like:

//class.h
#include "header.h"
...
#define COBJ_INTERFACE_IMPLEMENTATION_MODE
#include "interface.h"
#undef COBJ_INTERFACE_IMPLEMENTATION_MODE

So far everything looks ok, but what if "interface.h" uses a include-guard, and "header.h" includes "interface.h"?:

// header.h
#ifndef _HEADER_H_
#define _HEADER_H_ 

#include "interface.h"

#endif

//interface.h
#ifndef _INTERFACE_H_
#define _INTERFACE_H_ 

#define COBJ_INTERFACE_NAME interface
...
#include "cobj-interface-generator.h"

#endif

The result is, that "interface.h" is included at a time, where COBJ_INTERFACE_IMPLEMENTATION_MODE is not defined. Although it's included again in class.h, this is not effective, since the include guard (#ifndef _INTERFACE_H_), prevents the header to be processed again.

This seems to be related to #8, but even if COBJ_INTERFACE_IMPLEMENTATION_MODE is changed to an automatic check, the order still matters.