Working save_prompt, fixes to enable rotate station/building button and to compile with GCC

These patches enable window_save_prompt that asks for confirmation before leaving the game, loading another game or closing the application. The "save" button doesn't work yet, as I don't know which function handles this. The appearance of this prompt can be deactivated in the config to simplify testing the application. They also fix the MinGW compilation and enable the rotate station/building in the construction window.