Removed raw references to _WIN32; now just check to see if it is defined.

llvm-svn: 177291
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 91907cbe82b2d4e29d4bb83b7a7e75ff875d253e
diff --git a/src/thread.cpp b/src/thread.cpp
index 447eca7..1acb131 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -13,7 +13,7 @@
 #include "future"
 #include "limits"
 #include <sys/types.h>
-#if !_WIN32
+#if !defined(_WIN32)
 #if !__sun__ && !__linux__
 #include <sys/sysctl.h>
 #else