cmake: Rework how activities support is 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

@@ -575,12 +575,6 @@ if (KWIN_BUILD_TABBOX)
     )
 endif()
 
-if (KWIN_BUILD_ACTIVITIES)
-    set(kwin_SRCS ${kwin_SRCS}
-        activities.cpp
-    )
-endif()
-
 if (HAVE_LINUX_VT_H)
     set(kwin_SRCS ${kwin_SRCS}
         virtual_terminal.cpp
@@ -681,10 +675,6 @@ set(kwin_WAYLAND_LIBS
     ${CMAKE_THREAD_LIBS_INIT}
 )
 
-if (KWIN_BUILD_ACTIVITIES)
-    set(kwin_KDE_LIBS ${kwin_KDE_LIBS} KF5::Activities)
-endif()
-
 set(kwinLibs
     ${kwin_OWN_LIBS}
     ${kwin_QT_LIBS}
@@ -716,6 +706,11 @@ if (KWIN_BUILD_CMS)
     target_link_libraries(kwin lcms2::lcms2)
 endif()
 
+if (KWIN_BUILD_ACTIVITIES)
+    target_sources(kwin PRIVATE activities.cpp)
+    target_link_libraries(kwin KF5::Activities)
+endif()
+
 generate_export_header(kwin EXPORT_FILE_NAME kwin_export.h)
 
 if(CMAKE_SYSTEM MATCHES "FreeBSD")