Fix build when using -Werror=undef

Correct a couple places where macros were being checked when they
might not exist in some cases.

Fixes #3356.

src/google/protobuf/io/coded_stream.h

@@ -1434,7 +1434,7 @@ inline bool CodedInputStream::IsFlat() const {
 }  // namespace protobuf
 
 
-#if _MSC_VER >= 1300 && !defined(__INTEL_COMPILER)
+#if defined(_MSC_VER) && _MSC_VER >= 1300 && !defined(__INTEL_COMPILER)
   #pragma runtime_checks("c", restore)
 #endif  // _MSC_VER && !defined(__INTEL_COMPILER)
 

src/google/protobuf/stubs/hash.h

@@ -42,7 +42,7 @@
 #define GOOGLE_PROTOBUF_HAVE_HASH_SET 1
 
 // Use C++11 unordered_{map|set} if available.
-#if ((_LIBCPP_STD_VER >= 11) || \
+#if ((defined(_LIBCPP_STD_VER) && _LIBCPP_STD_VER >= 11) || \
     (((__cplusplus >= 201103L) || defined(__GXX_EXPERIMENTAL_CXX0X)) && \
     (__GLIBCXX__ > 20090421)))
 # define GOOGLE_PROTOBUF_HAS_CXX11_HASH