[CMake] How to set sensible Universal Binary options in OS X by default

[E. Wing]

So I would like to set the Universal Binary options as the default for
my project when on Apple. The issue is the options need to be
different for different versions of OS X.
In 10.4, I want to build ppc;i386. The SDK path must be set to 10.4u.
In 10.5, I want to build ppc;i386;ppc64;x86_64. I'm presuming the SDK
path should be set to a 10.5 thing.
In 10.3 and lower, Universal Binaries can't be built so none of this applies.

Is there an elegant way to do this? My not so elegant solution/thought
is to check OS X version numbers and case the thing. But how can I
detect the OS version number with CMake? The Darwin version number
isn't reliable for this case since Darwin may be upgraded
independently of the OS X version. (There is a corner case too where
it would help to know the Xcode version installed. 2.0 didn't have
Universal Binary support; the support was introduced in 2.1. But I
might not worry about this case if it's too hard.)

(It would be kind of nice if CMake would automatically pick these
defaults on the appropriate system. It's easier to for a user to
delete the options than figure out what the appropriate switches are.)

Thanks,
Eric