[HOpenGL] noticing freeglut vs not

I'm looking for a way to use MainLoopReturns when freeglut is present and not when it isn't. I could use some help. I figure that the implementation has some way to generate that error message, and maybe we could similarly just do a standard ungraceful exit if freeglut is missing. The great thing about MainLoopReturns is that it plays nicely with ghci. Closing a window returns to ghci for more fun. Some more info at http://trac.haskell.org/FieldTrip/ticket/8 . Any ideas? Thanks, - Conal