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/string.cpp b/src/string.cpp
index 40723e7..c71af4f 100644
--- a/src/string.cpp
+++ b/src/string.cpp
@@ -11,7 +11,7 @@
 #include "cstdlib"
 #include "cwchar"
 #include "cerrno"
-#if _WIN32
+#ifdef _WIN32
 #include "support/win32/support.h"
 #endif // _WIN32