add "support" for gtk 3.7 enum values, protect against older gtk

Hey!

This patch is necessary to compile against gtk 3.7. I kinda question the value of this structure, given its lack of robustness in the face of gtk updates, but that's your call. i've guarded against older GTK versions with a check against the appropriate GTK version macros; note that the enumeration values cannot be tested with ifdef, as they are not preprocessor tokens. Sorry for the gruesome solution.

This patch was developed in the course of packaging cinnamon 1.7.1 for SprezzOS 2. Please apply. Thanks!