Fix undef warning when building w/ clang or gcc on Windows

src/google/protobuf/stubs/port.h

@@ -51,7 +51,7 @@
   #if !defined(PROTOBUF_DISABLE_LITTLE_ENDIAN_OPT_FOR_TEST)
     #define PROTOBUF_LITTLE_ENDIAN 1
   #endif
-  #if _MSC_VER >= 1300 && !defined(__INTEL_COMPILER)
+  #if defined(_MSC_VER) && _MSC_VER >= 1300 && !defined(__INTEL_COMPILER)
     // If MSVC has "/RTCc" set, it will complain about truncating casts at
     // runtime.  This file contains some intentional truncating casts.
     #pragma runtime_checks("c", off)