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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@177291 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__config b/include/__config
index 8e39ad6..e50325a 100644
--- a/include/__config
+++ b/include/__config
@@ -96,7 +96,7 @@
 # endif
 #endif  // !defined(_LIBCPP_LITTLE_ENDIAN) || !defined(_LIBCPP_BIG_ENDIAN)
 
-#if _WIN32
+#ifdef _WIN32
 
 // only really useful for a DLL
 #ifdef _LIBCPP_DLL // this should be a compiler builtin define ideally...