[CMake] Ensuring not building for 64-bit

[ML]

Hi Mike,

> The double __LP64__ is making sure it (__LP64__) is both DEFINED and  
> TRUE.
>
> # if defined ((__APPLE__)) && defined(__LP64__) && __LP64__
> typedef CIconHandle  OSColorIcon;  // Mac CIcon
> #else

To to help me fine tune a bit..

Wouldn't (__APPLE__) be automatically defined if I am compiling on OS X?

Is there a place where I can see what others like __LP64__ are  
available?

-Jason