Fix parsing of empty strings in macro-processor

Those were treated as comments.

Incidentally, fix a similar issue in the Dynare parser (which is much less
problematic, since double quotes are not used in the Dynare language).

Closes DynareTeam/dynare#1621

src/DynareFlex.ll

@@ -96,8 +96,8 @@ DATE -?[0-9]+([YyAa]|[Mm]([1-9]|1[0-2])|[Qq][1-4]|[Ww]([1-9]{1}|[1-4][0-9]|5[0-2
 <INITIAL,DYNARE_STATEMENT,DYNARE_BLOCK,COMMENT,DATES_STATEMENT,LINE1,LINE2,LINE3>[\n]+       { yylloc->step(); }
 
  /* Comments */
-<INITIAL,DYNARE_STATEMENT,DYNARE_BLOCK,DATES_STATEMENT>["%"].*
-<INITIAL,DYNARE_STATEMENT,DYNARE_BLOCK,DATES_STATEMENT>["/"]["/"].*
+<INITIAL,DYNARE_STATEMENT,DYNARE_BLOCK,DATES_STATEMENT>%.*
+<INITIAL,DYNARE_STATEMENT,DYNARE_BLOCK,DATES_STATEMENT>\/{2}.*
 <INITIAL,DYNARE_STATEMENT,DYNARE_BLOCK,DATES_STATEMENT>"/*"   {comment_caller = YY_START; BEGIN COMMENT;}
 
 <COMMENT>"*/"        {BEGIN comment_caller;}

src/macro/MacroFlex.ll

@@ -71,7 +71,8 @@ CONT \\\\
   yylloc->step();
 %}
 
-<STMT,EXPR,FOR_BODY,THEN_BODY,ELSE_BODY>["/"]["/"].*
+ // Ignore inline comments
+<STMT,EXPR,FOR_BODY,THEN_BODY,ELSE_BODY>\/{2}.*
 
 <INITIAL>^{SPC}*@#{SPC}*includepath{SPC}+\"([^\"\r\n:;|<>]*){1}(:[^\"\r\n:;|<>]*)*\"{SPC}*{EOL} {
                               yylloc->lines(1);