Make no warning test stricter.

- Now it includes -Wextra except for unused-parameter.
- Removed gtest dependency

src/Makefile.am

@@ -922,16 +922,11 @@ no_warning_test.cc:
 	    echo "#include <$${FILE}>" >> no_warning_test.cc; \
 	  fi \
 	done
-	echo "#include <gtest/gtest.h>" >> no_warning_test.cc
-	echo "TEST(NoWarningTest, Empty) {}" >> no_warning_test.cc
+	echo "int main(int, char**) { return 0; }" >> no_warning_test.cc
 
-no_warning_test_LDADD = $(PTHREAD_LIBS) libprotobuf.la      \
-                        ../gmock/gtest/lib/libgtest.la      \
-                        ../gmock/gtest/lib/libgtest_main.la
-no_warning_test_CPPFLAGS = -I$(srcdir)/../gmock/gtest/include
+no_warning_test_LDADD = $(PTHREAD_LIBS) libprotobuf.la libprotoc.la
 no_warning_test_CXXFLAGS = $(PTHREAD_CFLAGS) $(PTHREAD_DEF) $(ZLIB_DEF) \
-                           -Wall -Werror -Werror=missing-declarations \
-                           -Werror=missing-field-initializers
+                           -Wall -Wextra -Werror -Wno-unused-parameter
 nodist_no_warning_test_SOURCES = no_warning_test.cc $(protoc_outputs)
 
 TESTS = protobuf-test protobuf-lazy-descriptor-test protobuf-lite-test \