Haddock built with the `in-ghc-tree` flag tries harder to find the GHC
lib folder and its own resources. This should make it possible to use
`in-ghc-tree`-built Haddock without having to specify the `-B` and
`--lib` options (just how you can use in-tree GHC without always
specifying the `-B` option).

The logic to do this relies on `getExecutablePath`, so we only get
this auto-detection on platforms where this function works.