fix regression about MOC compilations

The commit b05e1e9 was indeed the
cause of the regression. But the root cause was an incorrect
usage of `qt6_generate_moc`.

The addition of `TARGET ${plugin_name}` triggered the use of the
property `INCLUDE_DIRECTORIES` of the target.

See https://doc.qt.io/qt-6/qt-generate-moc.html#arguments

Polyhedron/demo/Polyhedron/polyhedron_demo_macros.cmake

@@ -19,7 +19,7 @@ include(${CGAL_MODULES_DIR}/CGAL_add_test.cmake)
       set(moc_file_name "")
     else()
       set(moc_file_name ${plugin_implementation_base_name}.moc )
-      qt6_generate_moc( ${plugin_implementation_base_name}.cpp "${CMAKE_CURRENT_BINARY_DIR}/${moc_file_name}" )
+      qt6_generate_moc( ${plugin_implementation_base_name}.cpp "${CMAKE_CURRENT_BINARY_DIR}/${moc_file_name}" TARGET ${plugin_name})
       add_file_dependencies( ${moc_file_name} "${CMAKE_CURRENT_SOURCE_DIR}/${plugin_implementation_base_name}.cpp" )
     endif()