[CMake] Ensuring not building for 64-bit

[ML]

Hi Sean,

>> I am running into some issues with some Carbon stuff that is not  
>> avail
>> under 64-bit, but is for 32-but.
>
> May I suggest you start by getting your stuff working in 32 bit first.
> With all the old Carbon stuff you use, 64 bit support will be more
> work.  And as you're apparently changing many things at once (Xcode to
> CMake, etc.), doing it one step at a time will be less painful.  Also,
> are you sure you actually need 64 bit support?  Are you working with
> large datasets?

Right, I just need 32-bit for now. The issue is that some how I think  
I am defaulting to compiling for 64-bit where the stuff is not  
available. I want to ensure compilation for 32-bit, force it sort of  
to say....

-Jason