Summary:
Ar was being checked twice prior to D3883 where I removed one of the checks
because the converted path was being overridden after the check because of
the second check for Ar. However the one in configure.ac was a target check
so I'm changing the path check to a target check now.

Test Plan: ./configure

Reviewers: angerman, austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, erikd

GHC Trac Issues: #14274

Differential Revision: https://phabricator.haskell.org/D4020