enable qt6 or qt5 for mapnik-viewer. android could be dropped

demo/viewer/CMakeLists.txt

@@ -1,8 +1,9 @@
 project(mapnik-viewer)
 
-find_package(Qt5 COMPONENTS Widgets REQUIRED)
+find_package(QT NAMES Qt6 Qt5 COMPONENTS Widgets REQUIRED)
+find_package(Qt${QT_VERSION_MAJOR} COMPONENTS Widgets REQUIRED)
 
-add_executable(mapnik-viewer 
+set(PROJECT_SOURCES
     about_dialog.cpp
     info_dialog.cpp
     layerdelegate.cpp
@@ -19,6 +20,23 @@ add_executable(mapnik-viewer
     mapnik_viewer.qrc
 )
 
+if(${QT_VERSION_MAJOR} GREATER_EQUAL 6)
+    qt_add_executable(mapnik-viewer
+        MANUAL_FINALIZATION
+        ${PROJECT_SOURCES}
+    )
+else()
+    if(ANDROID)
+        add_library(mapnik-viewer SHARED
+            ${PROJECT_SOURCES}
+        )
+    else()
+        add_executable(mapnik-viewer
+            ${PROJECT_SOURCES}
+        )
+    endif()
+endif()
+
 set_target_properties(mapnik-viewer PROPERTIES
     AUTOUIC_SEARCH_PATHS forms
     AUTORCC ON    
@@ -26,18 +44,23 @@ set_target_properties(mapnik-viewer PROPERTIES
     AUTOMOC ON
 )
 
-
 target_link_libraries(mapnik-viewer PRIVATE 
-    Qt5::Widgets
+    Qt${QT_VERSION_MAJOR}::Widgets
     mapnik::core
     mapnik::agg
     mapnik::mapnik
 )
+
 file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/viewer.ini 
 "[mapnik]
 plugins_dir=${PLUGINS_INSTALL_DIR}
 fonts/1/dir=${FONTS_INSTALL_DIR}"
 )
+
+if(QT_VERSION_MAJOR EQUAL 6)
+    qt_finalize_executable(mapnik-viewer)
+endif()
+
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/viewer.ini DESTINATION bin)
 mapnik_install(TARGET mapnik-viewer)