Check length of current directory on Windows in install.hs

[nponeccop]

My suggestion is:

• if we are on Windows
• if length <$> System.Directory.getCurrentDirectory is above a threshold (currently some 160 chars isn't buildable)
• create some long path and attempt to setCurrentDirectory there.
• If it succeeds then we have long paths enabled.
• if it fails print a warning and/or abort building

(a blind advice to enable long paths systemwide is not a good idea. Also note that for most of the users Haskell is a toy so the toy shouldn't break their systems used for more serious tasks)

[Anrock]

Sounds like a nice idea in general. Isn't it possible to detect long paths via some system variable or something? Creating and cding there sounds very ad-hoc.

[nponeccop]

The setting is only read during boot, and only by certain builds of Windows 10 and Windows Server 2016/2019.

Also, it's actually a bug in GHC older than 8.6.1 which the Windows setting manages to work around, see https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/win32-dlls.html#windows-file-paths

So by checking for a breakage using a broken GHC API we make it very robust against both these issues.

Oh, and my idea about cd actually seems wrong as SetCurrentDirectoryW keeps broken after long-paths setting is applied, so we need to check using another API which is fixed by long-paths (but I'm not sure).