Xcode: Resolve extra compiler (e.g qrc) dependencies

The Xcode generator relies on the generic makefile generator for extra
compilers such as qrc and moc, by generating makefiles that are then
executed as separate build steps in the Xcode build.

These makefiles are generated by entering a special mode in the Xcode
generator, in which case we _do_ want to resolve dependencies, so that
e.g. the files referenced inside a qrc file are added as dependencies
to the makefile rule that generates the qrc-cpp file.

Change-Id: I96bdcb165e9774a6328ae1980986fa2c6b00c6d9
Reviewed-by: Andy Shaw <andy.shaw@digia.com>