windows: Fix compilation with mingw-w64 so it uses correct printf implementations

Without this flag, mingw-w64 uses the MS implementations of snpintf and the likes.
This is not really a problem since they work with the the fixes provided for msvc,
but due to the way mingw-w64's stdio.h is structured we cannot get it to use the fixes.
modstruct: struct_calcsize: Fix case of uninitialized var.
Merge pull request #621 from stinos/migw-w64-fix

windows: Fix compilation with mingw-w64 so it uses correct printf implem...