cmake: Rework how tabbox is conditionally compiled into libkwin

In modern CMake code, it is highly advised to avoid using variables. One
should instead try to use targets and properties more extensively.

CMakeLists.txt

@@ -560,21 +560,6 @@ set(kwin_SRCS
     xwl/xwayland_interface.cpp
 )
 
-if (KWIN_BUILD_TABBOX)
-    include_directories(${Qt5Gui_PRIVATE_INCLUDE_DIRS})
-    set(kwin_SRCS ${kwin_SRCS}
-        tabbox/clientmodel.cpp
-        tabbox/desktopchain.cpp
-        tabbox/desktopmodel.cpp
-        tabbox/switcheritem.cpp
-        tabbox/tabbox.cpp
-        tabbox/tabbox_logging.cpp
-        tabbox/tabboxconfig.cpp
-        tabbox/tabboxhandler.cpp
-        tabbox/x11_filter.cpp
-    )
-endif()
-
 if (HAVE_LINUX_VT_H)
     set(kwin_SRCS ${kwin_SRCS}
         virtual_terminal.cpp
@@ -711,6 +696,21 @@ if (KWIN_BUILD_ACTIVITIES)
     target_link_libraries(kwin KF5::Activities)
 endif()
 
+if (KWIN_BUILD_TABBOX)
+    target_sources(kwin PRIVATE
+        tabbox/clientmodel.cpp
+        tabbox/desktopchain.cpp
+        tabbox/desktopmodel.cpp
+        tabbox/switcheritem.cpp
+        tabbox/tabbox.cpp
+        tabbox/tabbox_logging.cpp
+        tabbox/tabboxconfig.cpp
+        tabbox/tabboxhandler.cpp
+        tabbox/x11_filter.cpp
+    )
+    target_include_directories(kwin PRIVATE ${Qt5Gui_PRIVATE_INCLUDE_DIRS})
+endif()
+
 generate_export_header(kwin EXPORT_FILE_NAME kwin_export.h)
 
 if(CMAKE_SYSTEM MATCHES "FreeBSD")

autotests/tabbox/CMakeLists.txt

@@ -32,6 +32,7 @@ target_link_libraries(testTabBoxClientModel
 
     XCB::XCB
 )
+target_include_directories(testTabBoxClientModel PRIVATE ${Qt5Gui_PRIVATE_INCLUDE_DIRS})
 add_test(NAME kwin-testTabBoxClientModel COMMAND testTabBoxClientModel)
 ecm_mark_as_test(testTabBoxClientModel)
 
@@ -67,6 +68,7 @@ target_link_libraries(testTabBoxHandler
 
     XCB::XCB
 )
+target_include_directories(testTabBoxHandler PRIVATE ${Qt5Gui_PRIVATE_INCLUDE_DIRS})
 add_test(NAME kwin-testTabBoxHandler COMMAND testTabBoxHandler)
 ecm_mark_as_test(testTabBoxHandler)