Implement external functions

source/lexer.ll

@@ -55,6 +55,9 @@ begin                   {
 end                     {
                             return yy::parser::make_END_BLOCK(this->location);
                         }
+extern                  {
+                            return yy::parser::make_EXTERN(this->location);
+                        }
 const                   {
                             return yy::parser::make_CONST(this->location);
                         }