Macro-processor: when a variable is defined without a value, set it to “true”

Previously it was set to 1.

Also do the same change for variable defined on the command-line, for
consistency.

Closes: #43

src/DynareMain.cc

@@ -290,7 +290,7 @@ main(int argc, char **argv)
               equal_index != string::npos)
             defines.emplace_back(s.substr(2, equal_index-2), s.substr(equal_index+1));
           else
-            defines.emplace_back(s.substr(2), "1");
+            defines.emplace_back(s.substr(2), "true");
         }
       else if (s.substr(0, 2) == "-I")
         {

src/macro/Parser.yy

@@ -135,10 +135,7 @@ directive_one_line : INCLUDE expr
                    | INCLUDEPATH expr
                      { $$ = make_shared<IncludePath>($2, @$); }
                    | DEFINE symbol
-                     {
-                       auto tmp = make_shared<Real>("1", @$);
-                       $$ = make_shared<Define>($2, tmp, @$);
-                     }
+                     { $$ = make_shared<Define>($2, make_shared<Bool>(true, @$), @$); }
                    | DEFINE symbol EQUAL expr
                      { $$ = make_shared<Define>($2, $4, @$); }
                    | DEFINE function EQUAL expr