Fix OS default in Makefile.rules for Windows

[amccarth]

Summary

Broken off from D9033 per the discussion there. http://reviews.llvm.org/D9033

[zturner]

You can just commit this one, already lgtm'ed in the other thread :)

[amccarth]

http://reviews.llvm.org/rL235056

test/make/Makefile.rules

 
 # Uncomment line below for debugging shell commands
 # SHELL = /bin/sh -x
 
 THIS_FILE_DIR := $(shell dirname $(realpath $(lastword $(MAKEFILE_LIST))))/
 LLDB_BASE_DIR := $(THIS_FILE_DIR)../../
 
 #----------------------------------------------------------------------
-# If ARCH is not defined, default to x86_64.
 # If OS is not defined, use 'uname -s' to determine the OS name.
+#
+# uname on Windows gives "windows32", but most environments standardize
+# on "Windows_NT", so we'll make it consistent here.  When running
+# tests from Visual Studio, the environment variable isn't inherited
+# all the way down to the process spawned for make.
+#----------------------------------------------------------------------
+ifeq "$(OS)" ""
+  OS = $(shell uname -s)
+endif
+ifeq "$(OS)" "windows32"
+  OS = Windows_NT
+endif
+
+#----------------------------------------------------------------------
+# If ARCH is not defined, default to x86_64.
 #----------------------------------------------------------------------
 ifeq "$(ARCH)" ""
 ifeq "$(OS)" "Windows_NT"
   ARCH = x86
 else
   ARCH = x86_64
 endif
 endif
 
-ifeq "$(OS)" ""
-  OS = $(shell uname -s)
-endif
-
 #----------------------------------------------------------------------
 # CC defaults to clang.
 #
 # If you change the defaults of CC, be sure to also change it in the file
 # test/plugins/builder_base.py, which provides a Python way to return the
 # value of the make variable CC -- getCompiler().
 #
 # See also these functions: