Build with Cabal 1.21 (HEAD)

[UnkindPartition]

It would be nice to be able to build c-d-l with current Cabal HEAD. Right now, even if I pass --allow-newer, there's a compile error.

On the other hand, if I build it with Cabal 1.20, then it doesn't recognize the new config format:

cabal-dependency-licenses: user error (Saved package config file seems to be corrupt. Try re-running the 'configure' command.)

(Another workaround for me would be to build cabal-install-1.20 just for this purpose.)