fix: simplify build on Windows - #33

Hey,
This fixes a couple of issues I've had with building this crate on Windows:

1. The C code includes <windows.h> but clang does not include the Windows SDK directory automatically; The INCLUDE variable is where the Visual Studio Developer Shell places such paths to be included by MSVC, and so including it seems like a semi-standard way to resolve this issue without forcing users to set CFLAGS manually.
2. There are a lot of warnings coming from inside the Windows SDK headers, such as "warning: anonymous structs are a Microsoft extension" and "warning: non-portable path to file '"Windows.h"'; specified path differs in case from file name on disk"; these are printed in every build of my project which depends on this crate. Since as a user of this crate I don't have much to do about warnings anyway, I thought it would be a good idea to turn them off (maybe you still want to leave them on for your CI jobs?).

Let me know if you prefer to solve these in a different way.