RFR: JDK-8230653: jpackage error on macOS system without xcode

[Alexander Matveev]

Please review the jpackage fix for bug [1] at [2].

This is a fix for the JDK-8200758-branch branch of the open sandbox 
repository (jpackage).

- Error mentioned in bug report was due to missing Xcode when running 
SetFile. Fixed by not running SetFile if Xcode is not available. This 
tool used to set custom icon on DMG file and we do not consider it as 
real error. Verbose output will indicate that custom icon is skipped for 
DMG.
- Signing will fail if Xcode is not installed. Added check for Xcode 
availability if signing is requested. If Xcode is not available and 
signing is specified we will fail with error.

[1] https://bugs.openjdk.java.net/browse/JDK-8230653

[2] http://cr.openjdk.java.net/~almatvee/8230653/webrev.00

Thanks,
Alexander