[CMake] Distinguishing processor type for -march -mtune arguments

[James Bigler]

Is there a way to distinguish processor type more than what 
CMAKE_SYSTEM_PROCESSOR tells me?

Currently I can't tell the difference between a pentium M and pentium 4 
(both read i686) and various 64 bit architectures such as opteron versus 
pentium (both read x86_64).

I'm looking for a way to automate the -mtune argument to gcc on various 
systems.

Thanks,
James