Index: test/make/Makefile.rules
===================================================================
--- test/make/Makefile.rules
+++ test/make/Makefile.rules
@@ -30,8 +30,22 @@
 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"
@@ -41,10 +55,6 @@
 endif
 endif
 
-ifeq "$(OS)" ""
-	OS = $(shell uname -s)
-endif
-
 #----------------------------------------------------------------------
 # CC defaults to clang.
 #