Actually I take that back. I just added wchar.h into a source file from the glfw library to get at wcslen(), which is not in string.h as it is in other std library implementations and up pops that error again:
C:\PellesC\Include\wchar.h(267): error #2095: Expected type for parameter 4, but found 'FILE'.
C:\PellesC\Include\wchar.h(267): error #2048: Undeclared identifier 'FILE' (did you mean 'HFILE'?).
C:\PellesC\Include\wchar.h(268): error #2095: Expected type for parameter 3, but found 'FILE'.
C:\PellesC\Include\wchar.h(268): error #2048: Undeclared identifier 'FILE' (did you mean 'HFILE'?).
Has something been broken in wchar.h or windows.h between RC5 and RC6?
EDIT: Further to the above this error only occurs with __STDC_WANT_LIB_EXT2__ defined.