Building libraries with an in-place GHC

[Ryan Scott]

Hi Simon,

As #18018 says, you need to use head.hackage to reproduce it. This
page [1] contains a rather short tutorial on how to use it, but Ben is
currently drafting a more detailed tutorial on its use here [2]. This
is what I use whenever building any Hackage library with GHC HEAD.

Ryan S.
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[1] https://ghc.gitlab.haskell.org/head.hackage/
[2] https://gitlab.haskell.org/ghc/homepage/-/merge_requests/29