Are you saying that Homebrew's cmake correctly identifies the processor? Or that a cmake compiled from the official sources does so? I would rather expect this to be an upstream issue, if it is an issue at all (I don't know what the developers intend for CMAKE_SYSTEM_HOST_PROCESSOR to represent).
Note that "uname" does not necessarily tell you the best architecture to compile for, nor necessarily a valid architecture at all. On the PowerBook G4 I'm using today, "uname -m" says "Power Macintosh" whereas "uname -p" says "powerpc"; neither of those are valid PowerPC architectures (which would be something like "ppc" or "ppc7450"). On some early Intel Macs, "uname" might say "i386" because the computer uses a 32-bit kernel, while nevertheless being completely capable of running 64-bit usermode programs.
Why do you need to know the architecture? If you don't specify one, the compiler will use a reasonable default architecture.
