Alp Mestanogullari authored Apr 17, 2018 and Tamar Christina committed Apr 17, 2018

Summary:
This patch affects several files that affect how we detect mingw and perl
on Windows. The initial motivation is:

    https://github.com/snowleopard/hadrian/issues/564

where, with Hadrian building relocatable (non-inplace) GHCs, the current
detection mechanism falls short by e.g only trying $topdir/../mingw. But
in Hadrian, for reasons given in that issue, we would need to store e.g mingw
under $topdir/../../mingw except for binary distributions, where we want
to follow the existing structure, in which case $topdir/../mingw is correct. So
we need to support both, which is what this patch hopefully implements.

Test Plan: ./validate

Reviewers: Phyx, hvr, bgamari, erikd

Reviewed By: Phyx

Subscribers: snowleopard, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4598