Add WITH_QT6 option

- On systems with both qt5 and qt6 installed, cmake will use qt6 by default.

CMakeLists.txt

@@ -96,9 +96,14 @@ set(CMAKE_AUTOMOC ON)
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 set(QT_MIN_VER 5.9.5) # The version in Ubuntu 18.04 LTS
 # This is what Qt Creator 4.13.2 would generate
-find_package(QT NAMES Qt6 Qt5 COMPONENTS Core REQUIRED)
-find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Core Xml Widgets LinguistTools Svg Test Network REQUIRED)
-
+OPTION(WITH_QT6 "Enable Qt 6" OFF)
+if (WITH_QT6)
+    find_package(QT NAMES Qt6 COMPONENTS Core REQUIRED)
+    find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Core Xml Widgets LinguistTools Svg Test Network REQUIRED)
+else()
+    find_package(QT NAMES Qt5 COMPONENTS Core REQUIRED)
+    find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Core Xml Widgets LinguistTools Svg Test Network REQUIRED)
+endif()
 # Install paths depend on the build type and target platform
 setup_install_targets()