test.h

#define NONAMELESSUNION

#ifndef DUMMYSTRUCTNAME
#if defined(NONAMELESSUNION)
#define DUMMYSTRUCTNAME  s
#else
#define DUMMYSTRUCTNAME
#endif
#endif

typedef struct tagIMAWINDOWSHEADERSTRUCT {
    int whatever;
} IMAWINDOWSHEADERSTRUCT;

typedef struct tagIMAWINDOWSHEADERSTRUCTEX {
    IMAWINDOWSHEADERSTRUCT  DUMMYSTRUCTNAME;
    int                     someData;
} IMAWINDOWSHEADERSTRUCTEX;