Mention -MD and -MMD as passed through to the preprocessor in man-page

Update `gnu_prepro_help` for `-MD` and `-MMD`

Author: m-schmidt

doc/ccomp.1

@@ -110,7 +110,7 @@ Pass comma separated arguments in <args> to the preprocessor.
 Pass argument <arg> to the preprocessor.
 .
 .TP
-.BR \-C ", " \-CC ", " \-finput-charset ", " \-idirafter ", " \-imacros ", " \-iquote ", " \-isystem ", " \-M ", " \-MF ", " \-MG ", " \-MM ", " \-MP ", " \-MQ ", " \-MT  ", " \-nostdinc ", " \-P
+.BR \-C ", " \-CC ", " \-finput-charset ", " \-idirafter ", " \-imacros ", " \-iquote ", " \-isystem ", " \-M ", " \-MD ", " \-MF ", " \-MG ", " \-MM ", " \-MMD ", " \-MP ", " \-MQ ", " \-MT  ", " \-nostdinc ", " \-P
 For GNU backends these options are recognized by CompCert and passed through to the preprocessor.
 .
 .SS

driver/Frontend.ml

@@ -192,6 +192,9 @@ let gnu_prepro_help =
 {|  -M            Output a rule suitable for make describing the
                  dependencies of the main source file
   -MM            Like -M but do not mention system header files
+  -MD            Like -M but writes output to a .d file as a side-
+                 effect of compilation
+  -MMD           Like -MD but do not mention system header files
   -MF <file>     Specifies file <file> as output file for -M or -MM
   -MG            Assumes missing header files are generated for -M
   -MP            Add a phony target for each dependency other than