* ghcHEAD: bump to 8.11.20200403
* ghcHead: reduce diff vs. 8.10.1
dontAddExtraLibs was removed by accident (IMO) in ea19a8ed1e
* ghcHEAD: add ability to use system libffi
- enable nixpkgs' libffi
- minimise diffs against 8.10.1
- remove patching
* remove configure warning about --with-curses-includes
configure: WARNING: unrecognized options: --with-curses-includes