--- Find TopDir
--- for "installed" this is the root of GHC's support files
--- for "in-place" it is the root of the build tree
---
--- Plan of action:
--- 1. Set proto_top_dir
--- if there is no given TopDir path, get the directory
--- where GHC is running (only on Windows)
---
--- 2. If package.conf exists in proto_top_dir, we are running
--- installed; and TopDir = proto_top_dir
---
--- 3. Otherwise we are running in-place, so
--- proto_top_dir will be /...stuff.../ghc/compiler
--- Set TopDir to /...stuff..., which is the root of the build tree
---
--- This is very gruesome indeed
-