Add finalizer to call qt6_import_qml_plugins() automatically

Any target created by a call to qt6_add_executable() that also links to
the Qml target will now automatically call qt6_import_qml_plugins() in
qt6_finalize_executable() for scenarios that need it. This is only
relevant for static builds and only when not doing a top level Qt
superbuild.

The finalizers feature requires CMake 3.18. If using an earlier CMake
version, the project is still responsible for calling
qt6_import_qml_plugins() itself.

Fixes: QTBUG-86669
Pick-to: 6.1
Change-Id: I0f0b3f700ab6f1240b2373cb4155f52dc8991d2e
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/qml/Qt6QmlConfigExtras.cmake.in

@@ -0,0 +1,13 @@
+if(NOT QT_NO_CREATE_TARGETS AND
+   NOT "@BUILD_SHARED_LIBS@" AND              # Only needed if Qt was built statically
+   CMAKE_VERSION VERSION_GREATER_EQUAL 3.18)  # Finalizers require cmake_language(CALL)
+    set(target @QT_CMAKE_EXPORT_NAMESPACE@::Qml)
+    get_property(aliased_target TARGET ${target} PROPERTY ALIASED_TARGET)
+    if(aliased_target)
+        set(target "${aliased_target}")
+    endif()
+    set_property(TARGET ${target} PROPERTY
+        INTERFACE_QT_EXECUTABLE_FINALIZERS
+        qt@PROJECT_VERSION_MAJOR@_import_qml_plugins
+    )
+endif()

src/qml/Qt6QmlMacros.cmake

@@ -1088,10 +1088,21 @@ endfunction()
 
 include(CMakeParseArguments)
 
+# This function is called as a finalizer in qt6_finalize_executable() for any
+# target that links against the Qml library for a statically built Qt.
 function(qt6_import_qml_plugins target)
     if(QT6_IS_SHARED_LIBS_BUILD)
         return()
     endif()
+
+    # Protect against being called multiple times in case we are being called
+    # explicitly before the finalizer is invoked.
+    get_target_property(alreadyImported ${target} _QT_QML_PLUGINS_IMPORTED)
+    if(alreadyImported)
+        return()
+    endif()
+    set_target_properties(${target} PROPERTIES _QT_QML_PLUGINS_IMPORTED TRUE)
+
     set(options)
     set(oneValueArgs "PATH_TO_SCAN")
     set(multiValueArgs)