CPackWIX: Don't allow users to deselect the top-level feature (#15838)

The top-level WiX feature implicitly created by the WIX generator
represents the package as a whole.

As such it does not make sense to allow installer users to deselect
it from the installation.

Suggested-by: Mark Stijnman

Source/CPack/WiX/cmCPackWIXGenerator.cxx

@@ -482,6 +482,7 @@ bool cmCPackWIXGenerator::CreateWiXSourceFiles()
   featureDefinitions.BeginElement("Feature");
   featureDefinitions.AddAttribute("Id", "ProductFeature");
   featureDefinitions.AddAttribute("Display", "expand");
+  featureDefinitions.AddAttribute("Absent", "disallow");
   featureDefinitions.AddAttribute("ConfigurableDirectory", "INSTALL_ROOT");
 
   std::string cpackPackageName;