Fall back to include() method for finalizers with CMake 3.17 or earlier

Functionality is being added in other repos which require finalizers
to be run for non-static builds. That means we now need to ensure
finalizers always run even with CMake versions before
cmake_language() was added. The fallback method of writing to a file
and including it is slower, but that's better than missing important
functionality altogether.

Task-number: QTBUG-98545
Pick-to: 6.2
Change-Id: Ic8668c91c26d0c30de084b1b803032088c10fcf4
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/Qt6CoreMacros.cmake

@@ -702,11 +702,15 @@ function(_qt_internal_finalize_executable target)
 
     if(finalizers)
         if(CMAKE_VERSION VERSION_LESS 3.18)
-            # cmake_language() not available
-            message(WARNING
-                "Skipping module-specific finalizers for target ${target} "
-                "(requires CMake 3.18 or later)"
-            )
+            # cmake_language() not available, fall back to the slower method of
+            # writing a file and including it
+            set(contents "")
+            foreach(finalizer_func IN LISTS finalizers)
+                string(APPEND contents "${finalizer_func}(${target})\n")
+            endforeach()
+            set(finalizer_file "${CMAKE_CURRENT_BINARY_DIR}/.qt/finalize_${target}.cmake")
+            file(WRITE ${finalizer_file} "${contents}")
+            include(${finalizer_file})
         else()
             foreach(finalizer_func IN LISTS finalizers)
                 cmake_language(CALL ${finalizer_func} ${target})