add_custom_command: Evaluate generator expressions in DEPENDS

Rely on evaluation in cmCustomCommandGenerator for the generators.

When tracing target dependencies, depend on the union of dependencies
for all configurations.

Source/cmCustomCommandGenerator.h

@@ -28,6 +28,8 @@ class cmCustomCommandGenerator
   bool OldStyle;
   bool MakeVars;
   cmGeneratorExpression* GE;
+  mutable bool DependsDone;
+  mutable std::vector<std::string> Depends;
 public:
   cmCustomCommandGenerator(cmCustomCommand const& cc,
                            const std::string& config,