Use CPP hackery to make it compile with ghc-8.0 and ghc 7.10. If
ghc-7.10 works, I assume earlier supported versions of GHC also
work. All tests pass with both GHC versions.
Unfortunately, the TH changes force changes in the type signature
of Yesod.Routes.TH.RenderRoute.mkRouteCons from:
mkRouteCons :: [ResourceTree Type] -> ([Con], [Dec])
to
mkRouteCons :: [ResourceTree Type] -> Q ([Con], [Dec])
and I can't see a way around that.
