misc: Fix compile warnings about undefined _MSC_VER

commit: a500fc4
diff --git a/include/vulkan.h b/include/vulkan.h
index a38b848..b11c0a3 100644
--- a/include/vulkan.h
+++ b/include/vulkan.h
@@ -44,7 +44,7 @@
 #define VK_API_VERSION VK_MAKE_VERSION(0, 175, 0)
 
 
-#if defined(__cplusplus) && (_MSC_VER >= 1800 || __cplusplus >= 201103L)
+#if defined(__cplusplus) && ((defined(_MSC_VER) && _MSC_VER >= 1800) || __cplusplus >= 201103L)
     #define VK_NULL_HANDLE nullptr
 #else
     #define VK_NULL_HANDLE 0