Recognize shells reporting as MINGW under windows.

Summary:
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
file names.

This fixes #14654

Test Plan: make

Reviewers: O26 nofib, Phyx, bgamari

Reviewed By: Phyx, bgamari

Subscribers: bgamari, Phyx

GHC Trac Issues: #14654

Differential Revision: https://phabricator.haskell.org/D4297

mk/boilerplate.mk

@@ -23,7 +23,7 @@ CONTEXT_DIFF_RAW = diff -U 1
 EXECUTABLE_FILE = chmod +x
 
 # Windows MSYS specific settings
-ifeq ($(shell uname -s | grep -c MSYS), 1)
+ifeq ($(shell uname -s | grep -c 'MSYS\|MINGW'), 1)
 	exeext=.exe
 	CONTEXT_DIFF=$(CONTEXT_DIFF_RAW) --strip-trailing-cr
 else