path to ghc no longer works in the 8.8.1 build

Summary

The flags --with-ghc no longer works to build ghc (which breaks ghcup) and the suggested replacement to use GHC does not work.

I'd personally like to see --with-ghc return as it will require a workaround in ghcup (and probably other packaging tools) to support the new system anyway.

Steps to reproduce

$ curl -L --fail -O https://downloads.haskell.org/~ghc/8.8.1/ghc-8.8.1-src.tar.xz
$ cd ghc-8.8.1
$ ./boot
$ GHC=ghc-8.6.5 ./configure --prefix=$HOME/.ghcup/ghc/8.8.1

...
checking for gfind... no
checking for find... /usr/bin/find
checking for sort... /usr/bin/sort
checking for GHC Git commit id... given 9c787d4d24f2b515934c8503ee2bbd7cfac4da20
checking for ghc... no
configure: error: GHC is required.

Also, a minor point, but the 8.8.1 tarball required me to install alex/happy whereas this has not been needed for previous versions.

Expected behavior

Compile ghc.

Environment

Archlinux.