port diagnose and xcode

[Joshua Root]

> I truly appreciate everyone who maintains things for MP - couldn’t live without this stuff.  My initial query was just trying to understand whether ‘port diagnose’ was telling me something I should be concerned about.  I think the answer was ‘no’.
The message could be improved for that case, certainly. All that check 
does currently is print the message that started this discussion if your 
currently installed version of Xcode is not in the list of versions 
known to work on your OS version: 
<https://github.com/macports/macports-ports/blob/master/_resources/macports1.0/xcode_versions.ini>

If no version of Xcode is installed, it should say something along the 
lines of "You don't have Xcode installed, so you will not be able to 
build the subset of ports that build using Xcode."

It should also check the CLT version but currently doesn't.

- Josh