We need to set CMAKE_CXX_SCAN_FOR_MODULES too

Author: ClausKlein

CMakeLists.txt

@@ -1,12 +1,17 @@
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+if(NOT DEFINED CMAKE_CXX_STANDARD)
+    set(CMAKE_CXX_STANDARD 20)
+endif()
+
 # TODO(CK): set(CMAKE_CXX_MODULE_STD 1)
 if(CMAKE_CXX_MODULE_STD)
     set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
         "d0edc3af-4c50-42ea-a356-e2862fe7a444"
     )
     set(CMAKE_CXX_STANDARD 26)
-    set(CMAKE_CXX_STANDARD_REQUIRED OFF)
 endif()
+set(CMAKE_CXX_STANDARD_REQUIRED OFF)
+set(CMAKE_CXX_SCAN_FOR_MODULES 1)
 
 cmake_minimum_required(VERSION 3.28...4.0)