[CMake] XCode 2.1 partial support

[William A. Hoffman]

>I know on the Visual Studio side of CMake, there have been requests  
>to allow users to generate projects for different versions of Visual  
>Studio that may not exist on your computer. The differences between  
>Xcode 1.5/2.0 and 2.1 might be minimal enough that this could be  
>feasible so maybe it might be worth considering having the ability to  
>generate Xcode projects for other versions. Maybe -GXcode will do the  
>one on your system, but you can override using an explicit -GXcode15  
>or -GXcode21?
The big difference is that with visual studio, they do allow multiple version
of visual studio to be on the same machine, and many developers do have more
than one.   Since Xcode prevents this, then all we have to do is figure out
which one is on the machine, and generate for that Xcode.   So, I don't see
the need or even use for -GXcode15 since if you give it the wrong one, it won't
even work, because all the try/compile stuff will fail.

-Bill