Disable the asynchronous implementation when building with GHC > 7.8

The system event manager of that release has one-shot semantics which
breaks the usb library which assumes multi-shot semantics.

A fix is being developed which should hopefully be ready for a next
GHC release. See:

  http://www.haskell.org/pipermail/ghc-devs/2014-October/006682.html

When that's done we'll adapt usb to use it.