Tamar Christina authored Mar 06, 2017 and Ben Gamari committed Mar 06, 2017

D3155 added pthread by default to GHCi.  However this was only tested
using something running in an msys2 shell. As it turns out it's been
picking up pthread in the environment and not the inplace gcc.

This results in an error when using ghci outside of msys2.

Test Plan: start cmd, start ghc-stage2 --interactive

Reviewers: austin, hvr, bgamari, dfeuer

Reviewed By: dfeuer

Subscribers: dfeuer, thomie

Differential Revision: https://phabricator.haskell.org/D3279