Detect changes to XCConfig files during target build up-to-date checking

[Daniel Lacasse]

As a user building Xcode target using Gradle, I want the task representing the Xcode target to be out-of-date during an incremental build when changes to .xcconfig files so that I can receive accurate feedback.

At this stage, we should focus on the first level, the file pointed by baseConfigurationReference. We need to check and see if the file contains #include and force the whole task to be out-of-date to ensure correctness. We can refine this behaviour later. Most .xcconfig don't include other files.

Acceptance Criteria

• When the xcconfig file pointed to by baseConfigurationReference changes, the task is out-of-date.
• When the xcconfig file has #include (required/optional) the task is always out-of-date.