Do Not Define GOOGLE_FALLTHROUGH_INTENDED on GCC Without Attribute Support

The definition of GOOGLE_FALLTHROUGH_INTENDED assumed that
[[gnu::fallthrough]] was always available on GCC 7+. While
gnu::fallthrough is supported on GCC 7+, C++ attributes themselves are
not supported on GCC if the C++ standard is before C++11. As a fix,
the guard on this definition has been expanded to include a check for
C++ attribute support. The form of this check is the one recommended
by GCC at https://gcc.gnu.org/projects/cxx-status.html .

src/google/protobuf/stubs/port.h

@@ -243,7 +243,7 @@ static const uint64 kuint64max = GOOGLE_ULONGLONG(0xFFFFFFFFFFFFFFFF);
      __has_cpp_attribute(clang::fallthrough)
 #  define GOOGLE_FALLTHROUGH_INTENDED [[clang::fallthrough]]
 # endif
-#elif defined(__GNUC__) && __GNUC__ > 6
+#elif defined(__GNUC__) && __GNUC__ > 6 && __cpp_attributes >= 200809
 # define GOOGLE_FALLTHROUGH_INTENDED [[gnu::fallthrough]]
 #endif