[CMake] CPack issue with PackageMaker

[Timenkov Yuri]

Hi everyone.
This is just FYI.

I'm started to playing with CPack (2.4) for Apple (Tiger 10.4, PackageMaker 
v2.1.1), and encountered very strange error:
CPack: Compress package

The package could not be created because of the following errors:
No package identifier specified.

While investigating causes of such strange behavior of PackageMaker I came to 
idea that cpack generates invalid XML file Info.plist. And does it because 
CPACK_PACKAGE_RELOCATABLE variable is not set. Moreover, it should be set to 
string value in lowercase.

Regards, Yuri