Actually print __GLASGOW_HASKELL__ in the comments

Currently, the Haddock pages for `vector-th-unbox` have a bug in which the `__GLASGOW_HASKELL__` macro gets expanded to an actual number within comments (see [here](http://hackage.haskell.org/package/vector-th-unbox-0.2.1.0/docs/Data-Vector-Unboxed-Deriving.html) for example, note `#if 706 == 704`). I attempt to solve this CPP oddity... with more CPP oddities. I know that both `gcc` and `clang` support `push_macro` and `pop_macro`, so this should be portable.