Ben Gamari authored Sep 04, 2016 and Ben Gamari committed Sep 05, 2016

As reported in #12555 this code was terribly broken. Sadly, Autoconf was
none-the-wiser. Thanks to @rwbarton for pointing this out.

Test Plan: Test with libdw version newer and older and 0.158

Reviewers: hvr, austin, rwbarton

Reviewed By: rwbarton

Subscribers: thomie, rwbarton, erikd

Differential Revision: https://phabricator.haskell.org/D2510

GHC Trac Issues: #12555