Ben Gamari authored Apr 25, 2017 and Ben Gamari committed Apr 25, 2017

This replaces the --with-* configure flags with the usual autoconf
environment variables, as suggested by #13583.

Test Plan: Configure on various platforms

Reviewers: hvr, trofi, thomie, austin

Reviewed By: trofi

Subscribers: rwbarton, erikd

GHC Trac Issues: #13583

Differential Revision: https://phabricator.haskell.org/D3499