Error when using -main without -interp

Outside of -interp mode, -main has no (known) effect but could be
confused for a linker option that sets the program's entrypoint, say.
It's safer to reject the option.

driver/Driver.ml

@@ -412,6 +412,8 @@ let _ =
       fatal_error no_loc "ambiguous '-o' option (multiple source files)";
     if !num_input_files = 0 then
       fatal_error no_loc "no input file";
+    if not !option_interp && !main_function_name <> "main" then
+      fatal_error no_loc "option '-main' requires option '-interp'";
     let linker_args = time "Total compilation time" perform_actions () in
     if not (nolink ()) && linker_args <> [] then begin
       linker (output_filename_default "a.out") linker_args