Help with class constants

Some changes in GObject make am error in my app. There is some users 
with the gtk 2.8 and others with 2.10, the 2.8 uses the beta and 2.10 
the cvs.

My question: how can I know if a class constant is defined?
If I test:

if (isset(Gtk::TYPE_STRING))

And the constant do not exist I get a parser error(fatal) . There is any 
way to do this?

Thanks.

Fernando Correa da Conceição