CPack/WiX: Allow multiple empty top-level components

Avoid generating empty / conflicting directory components for
the installation root which is shared across components and
(except for the degenerate case of an empty installer)
will always be created implicitly.

Source/CPack/WiX/cmCPackWIXGenerator.cxx

@@ -1015,6 +1015,10 @@ void cmCPackWIXGenerator::AddDirectoryAndFileDefinitions(
 
   if (emptyDirectory) {
     createDirectory = true;
+
+    if (directoryId == "INSTALL_ROOT") {
+      return;
+    }
   }
 
   if (directoryInstalledFile) {

Tests/RunCMake/CPack_WIX/3-AppWiX-cpack-WIX-stdout.txt

@@ -3,6 +3,8 @@ CPack: Install projects
 CPack: - Install project: CPackWiXGenerator \[Release\]
 CPack: -   Install component: applications
 CPack: -   Install component: applications2
+CPack: -   Install component: empty1
+CPack: -   Install component: empty2
 CPack: -   Install component: extras
 CPack: -   Install component: headers
 CPack: -   Install component: libraries

Tests/RunCMake/CPack_WIX/4-AppWiX-cpack-WIX-stdout.txt

@@ -3,6 +3,8 @@ CPack: Install projects
 CPack: - Install project: CPackWiXGenerator \[Release\]
 CPack: -   Install component: applications
 CPack: -   Install component: applications2
+CPack: -   Install component: empty1
+CPack: -   Install component: empty2
 CPack: -   Install component: extras
 CPack: -   Install component: headers
 CPack: -   Install component: libraries

Tests/RunCMake/RunCPack/AppWiX/CMakeLists.txt

@@ -33,6 +33,9 @@ install(FILES mylib.h "file with spaces.h"
   DESTINATION include
   COMPONENT headers)
 
+install(CODE "" COMPONENT "empty1")
+install(CODE "" COMPONENT "empty2")
+
 set(CPACK_GENERATOR "WIX")
 
 set(CPACK_PACKAGE_NAME "MyLib")