Add -Werror=missing-declarations to test builds

Treat missing declarations as an error for test builds, to prevent
future updates which add functions without properly declaring them.
This will prevent broken builds in more restrictive build environments.

src/Makefile.am

@@ -925,7 +925,7 @@ no_warning_test_LDADD = $(PTHREAD_LIBS) libprotobuf.la      \
                         ../gmock/gtest/lib/libgtest_main.la
 no_warning_test_CPPFLAGS = -I$(srcdir)/../gmock/gtest/include
 no_warning_test_CXXFLAGS = $(PTHREAD_CFLAGS) $(PTHREAD_DEF) $(ZLIB_DEF) \
-                           -Wall -Wconversion -Werror
+                           -Wall -Wconversion -Werror -Werror=missing-declarations
 nodist_no_warning_test_SOURCES = no_warning_test.cc $(protoc_outputs)
 
 TESTS = protobuf-test protobuf-lazy-descriptor-test protobuf-lite-test \