patch 8.1.0128: building with MinGW does not work out-of-the-box

Problem:    Building with MinGW does not work out-of-the-box.
Solution:   Add instructions for MSYS2.  Set default WINVER.  Add batch files
            to set $PATH for MSYS2.
diff --git a/src/Make_cyg_ming.mak b/src/Make_cyg_ming.mak
index cdec101..16cd712 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -75,9 +75,10 @@
 # Set to yes to enable OLE support.
 OLE=no
 
-# Set the default $(WINVER) to make it work with WinXP.
+# Set the default $(WINVER).  Use 0x0501 to make it work with WinXP.
 ifndef WINVER
-WINVER = 0x0501
+# WINVER = 0x0501
+WINVER = 0x0600
 endif
 
 # Set to yes to enable Cscope support.