What I have learned is that pelles c can handle nameless unions in structs (a c11 feature), and orange c can not. It uses a dummy union name 'u'. I have seen this for example in:
typedef struct {
HTREEITEM hParent;
HTREEITEM hInsertAfter;
#if (_WIN32_IE >= 0x0400)
union {
TVITEMEX itemex;
TVITEM item;
} DUMMYUNIONNAME;
#else
TVITEM item;
#endif
} TVINSERTSTRUCT, *LPTVINSERTSTRUCT;