Recognize shells reporting as MINGW under windows.
My msys instance at least reports MINGW instead of MSYS for uname.
This commit adjusts the regex to match either one.
Without this some errors occur because of missing .exe extensions in
This fixes Trac #14654
Test Plan: make
Reviewers: O26 nofib, Phyx, bgamari
Reviewed By: Phyx, bgamari
Subscribers: bgamari, Phyx
GHC Trac Issues: Trac #14654
Differential Revision: https://phabricator.haskell.org/D4297