Fix some compilation warnings with MSVC 2015

MSVC 2015 already defines vsnprintf() and va_copy().

src/windows_port.cc

@@ -44,6 +44,7 @@
 
 // These call the windows _vsnprintf, but always NUL-terminate.
 #if !defined(__MINGW32__) && !defined(__MINGW64__)  /* mingw already defines */
+#if !(defined(_MSC_VER) && _MSC_VER >= 1900)  /* msvc 2015 already defines */
 
 #ifdef _MSC_VER
 #  pragma warning(push)
@@ -59,8 +60,6 @@ int safe_vsnprintf(char *str, size_t size, const char *format, va_list ap) {
 #  pragma warning(pop)
 #endif
 
-#if _MSC_VER < 1900  // msvs 2015 finally includes snprintf
-
 int snprintf(char *str, size_t size, const char *format, ...) {
   int r;
   va_list ap;
@@ -70,6 +69,5 @@ int snprintf(char *str, size_t size, const char *format, ...) {
   return r;
 }
 
-#endif
-
+#endif  /* if !(defined(_MSC_VER) && _MSC_VER >= 1900)  */
 #endif  /* #if !defined(__MINGW32__) && !defined(__MINGW64__) */

src/windows_port.h

@@ -63,12 +63,14 @@
  * name vsnprintf, since windows defines that (but not snprintf (!)).
  */
 #if !defined(__MINGW32__) && !defined(__MINGW64__)  /* mingw already defines */
+#if !(defined(_MSC_VER) && _MSC_VER >= 1900)  /* msvc 2015 already defines */
 extern GFLAGS_DLL_DECL int snprintf(char *str, size_t size,
                                        const char *format, ...);
 extern int GFLAGS_DLL_DECL safe_vsnprintf(char *str, size_t size,
                                              const char *format, va_list ap);
 #define vsnprintf(str, size, format, ap)  safe_vsnprintf(str, size, format, ap)
 #define va_copy(dst, src)  (dst) = (src)
+#endif
 #endif  /* #if !defined(__MINGW32__) && !defined(__MINGW64__) */
 
 #ifdef _MSC_VER