Protect a new death test with GTEST_HAS_DEATH_TEST, since MinGW doesn't support it.

src/google/protobuf/extension_set_unittest.cc

@@ -465,6 +465,8 @@ TEST(ExtensionSetTest, SpaceUsedExcludingSelf) {
   }
 }
 
+#ifdef GTEST_HAS_DEATH_TEST
+
 TEST(ExtensionSetTest, InvalidEnumDeath) {
   unittest::TestAllExtensions message;
   EXPECT_DEBUG_DEATH(
@@ -473,6 +475,8 @@ TEST(ExtensionSetTest, InvalidEnumDeath) {
     "IsValid");
 }
 
+#endif GTEST_HAS_DEATH_TEST
+
 }  // namespace
 }  // namespace internal
 }  // namespace protobuf