re-work code gen so that it doesnt re-build all the time

Author: enetheru

cmake/GodotCPPModule.cmake

@@ -107,15 +107,16 @@ function(
     # Strip newlines and whitespace to make it a one-liner.
     string(REGEX REPLACE "\n *" " " PYTHON_SCRIPT "${PYTHON_SCRIPT}")
 
-    add_custom_target(
-        generate_bindings
+    add_custom_command(
+        OUTPUT ${GENERATED_FILES_LIST}
         COMMAND "${Python3_EXECUTABLE}" "-c" "${PYTHON_SCRIPT}"
-        BYPRODUCTS ${GENERATED_FILES_LIST}
+        MAIN_DEPENDENCY ${GODOTCPP_GDEXTENSION_API_FILE}
+        DEPENDS ${godot-cpp_SOURCE_DIR}/binding_generator.py
         WORKING_DIRECTORY ${godot-cpp_SOURCE_DIR}
         COMMENT "Generating bindings"
         VERBATIM
-        SOURCES ${godot-cpp_SOURCE_DIR}/binding_generator.py ${GODOTCPP_GDEXTENSION_API_FILE}
     )
+    add_custom_target(generate_bindings DEPENDS ${GENERATED_FILES_LIST})
     set_target_properties(generate_bindings PROPERTIES FOLDER "godot-cpp")
 endfunction()