Update CMakeLists.txt to support both Qt5 and Qt6. Add option ENABLE_QT6

plugins/samplesource/testsource/CMakeLists.txt

@@ -33,7 +33,7 @@ if(NOT SERVER_MODE)
     )
 
     set(TARGET_NAME inputtestsource)
-    set(TARGET_LIB "Qt5::Widgets")
+    set(TARGET_LIB "Qt::Widgets")
     set(TARGET_LIB_GUI "sdrgui")
     set(INSTALL_FOLDER ${INSTALL_PLUGINS_DIR})
 else()
@@ -48,7 +48,7 @@ add_library(${TARGET_NAME} SHARED
 )
 
 target_link_libraries(${TARGET_NAME}
-        Qt5::Core
+        Qt::Core
         ${TARGET_LIB}
 	sdrbase
 	${TARGET_LIB_GUI}