[CMake] Properly Detecting Win64

[Rolf Eike Beer]

>> Maybe you can base the package name by testing the generator name.
>
> I need to do that to e.g. get a useful build naming in CTest scripts. At
> that point CMAKE_SIZEOF_VOID_P is not available anyway. And I think that
> using this is just a bad idea. How do you decide if it's IA64 or AMD64?

[...]

And if you are inside the build you have also CMAKE_CL_64. At least when
using NMake. IIRC it is not set when using the Visual Studio Generators so
you are back to the previously posted solution.

Eike