Fix build failure with mingw-w64
Fix build failure with mingw-w64

mingw-w64 has fseeko and respects _FILE_OFFSET_BITS.  Give stdio.h a
chance to define fseeko before we do it.  Fixes redefinition errors
inside stdio.h.

openslide/openslide,openslide/openslide,openslide/openslide,openslide/openslide