[CMake] XCode 2.1 partial support

[William A. Hoffman]

>
>>So, I have a question for the Mac folks out there.   Is it possible for
>>two version of Xcode to be installed on the same machine?  Can you have
>>Xcode 1.5 and Xcode 2.1 on the same machine?
>
>No and No.  Xcode is tied closely to the OS.  2.0/2.1 requires OS 10.4
>and will not work on 10.3.  1.5 requires 10.3 and will not work on 10.4.

OK, so I think the Xcode strategy should be as follows.

1. The generator name should be Xcode and not Xcode15 and Xcode2.
Xcode 1.5 and Xcode 2.1 or greater will be supported by CMake.  
2.0 will not be a supported version of Xcode.

2. We will determine the version of Xcode by looking at the "/Developer/Applications/Xcode.app/Contents/version.plist" file.
Based on that we will generate the correct version of Xcode files
for that machine.

3. As a temporary fix, we will generate Xcode 2.1 compatible files
with minor tweaks to the current Xcode 1.5 generator similar to the patch
posted by  Mathieu.   Basically, tweak the paths to know about the
configuration directories and use the correct arguments to xcodebuild
for try-compiles.   At some point, we will create an actual 2.1 generator so
that it will not have to be converted by Xcode prior to use.

-Bill