Pass options -MD and -MMD to the preprocessor

Like the other GNU `-M` options.

Fixes: #553

Author: xavierleroy

driver/Frontend.ml

@@ -159,6 +159,8 @@ let gnu_prepro_actions = [
   Exact "-MP", Self gnu_prepro_opt;
   Exact "-MT", String (gnu_prepro_opt_key "-MT");
   Exact "-MQ", String (gnu_prepro_opt_key "-MQ");
+  Exact "-MD", Self gnu_prepro_opt;
+  Exact "-MMD", Self gnu_prepro_opt;
   Exact "-nostdinc", Self (fun s -> gnu_prepro_opt s; use_standard_headers := false);
   Exact "-imacros", String (gnu_prepro_opt_key "-imacros");
   Exact "-idirafter", String (gnu_prepro_opt_key "-idirafter");