cmTarget: Make the source files depend on the config.
Disallow the use of config-specific source files with the Visual Studio and Xcode generators. They don't have any way to represent the condition currently. Use the same common-config API in cmQtAutoGenerators. While it accepts config-specific files, it doesn't have to support multiple configurations yet. Loop over the configs in cmTargetTraceDependencies and cmGlobalGenerator::WriteSummary and consume all source files. Loop over the configs in cmComputeTargetDepends and compute the object library dependencies for each config.