use custom command rather than run for generator script

CMakeLists.txt

@@ -17,6 +17,7 @@ target_sources(counting
 	PRIVATE
 		labs/counting/main.cpp
 )
-exec_program(python ${CMAKE_CURRENT_BINARY_DIR}
-	ARGS ${CMAKE_CURRENT_SOURCE_DIR}/labs/counting/generate.py
+add_custom_command( TARGET counting
+	COMMAND python ${CMAKE_CURRENT_SOURCE_DIR}/labs/counting/generate.py
+	WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
 )