Preprocessor: fix problem with "%" sign inside string in native statements

DynareFlex.ll

@@ -580,7 +580,9 @@ string eofbuff;
 
  /* Add the native statement */
 <NATIVE>{
-  [^/%*\n\.]*                 |
+  [^/%*\n\.\'\"]*             |
+  \'[^\'\n]*\'                |
+  \"[^\"\n]*\"                |
   \.{1,2}                     |
   "*"                         |
   "/"                         { yymore(); eofbuff = string(yytext); }