OK.
In fact with MySys2 it is mingw and there is no _MSC_VER macro definition neither ...
So it definitely can't work.

Is there any reasons why MINGW is not wanted for the windows version of the function ?
For example with libtif I've faced to the same issue but the windows function is ok with mingw:

extern TIFF* TIFFOpen(const char*, const char*);
# ifdef __WIN32__
extern TIFF* TIFFOpenW(const wchar_t*, const char*);
# endif /* __WIN32__ */

Cheers,
Cyril