#453 [lxsession]fix vala compile

[marguerite]

We add a option to "enable gtk3".

But actually no matter gtk2 or gtk3, we explicitly use gtk+-2.0 pkg to generate c files, and compile using gtk3.

That's simply wrong.

It'll generate some GtkObject in c file, while doesn't exist anymore in gtk3(Now GObject).

[marguerite]

fix vala pkg "gtk+-2.0" to a variable in configure.ac

[gilir]

Thanks for the patch. I adpated it and push it to git.