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Makefile.mingw already

Makefile.mingw already defines -DLIBRAW_NODLL, so disabling DllDef (and, of course, DLL builds):

DllDef is defined in libraw_types.h:
#ifdef WIN32
#ifdef LIBRAW_NODLL
#define DllDef
#else
#ifdef LIBRAW_BUILDLIB
#define DllDef __declspec(dllexport)
#else
#define DllDef __declspec(dllimport)
#endif
#endif
#else
#define DllDef
#endif

-- Alex Tutubalin