mingw + offsetof() doesn't work

//
// This test was designed to test the asOBJ_IS_COMPLEX flag
//
// Author: Andreas Jönsson
//

#include "angelscript.h"
#include <stdio.h>
#include <stddef.h> // offsetof(class, property)

#define TESTNAME "TestComplex"

// A complex class is a class that has a defined constructor or 
// destructor, or an overridden assignment operator. Compilers
// normally treat these classes differently in that they are
// returned by reference even though they are small enough to 
// fit in registers. This is because of the need of exception 
// handling in case something goes wrong.

class CComplexClass
{
public:
	CComplexClass() { a = 0xDEADC0DE; }
	
	unsigned long a;
};

static CComplexClass cfunction()
{
	CComplexClass c;

	return c;
}

static CComplexClass c;

class COutStream : public asIOutputStream
{
public:
	void AS_CALL Write(const char *text) {printf(text);}
};

bool TestComplex()
{
	bool ret = false;

	asIScriptEngine *engine = asCreateScriptEngine(ANGELSCRIPT_VERSION);

	// If asOBJ_IS_COMPLEX is not specified AngelScript will try to determine the 
	// type of object by looking at what behaviours are registered for the object.
	// Since we are not registering any behaviours this time AngelScript would fail.
	engine->RegisterObjectType("complex", sizeof(CComplexClass), asOBJ_IS_COMPLEX);
	engine->RegisterObjectProperty("complex", "bits a", offsetof(CComplexClass, a));
	
	engine->RegisterGlobalProperty("complex c", &c);
	engine->RegisterGlobalFunction("complex cfunction()", asFUNCTION(cfunction), asCALL_CDECL);

	c.a = 0;

	COutStream out;
	engine->ExecuteString(0, "c = cfunction();", &out, 100, 0);

	if( c.a != 0xDEADC0DE )
	{
		printf("%s: Failed to assign complex object returned from function\n", TESTNAME);
		ret = true;
	}

	engine->Release();

	return ret;
}