Makefile.am: fix parallel build of t/generated-code2/cxx-generate-packed-data

The object file name is not what is expected when building with
automake's "subdir-objects" option. Rather than expecting automake to
generate a specific filename, we simply make the generated header a
dependency of all objects for cxx-generate-packed-data.

(Issue #156, #169.)

Makefile.am

@@ -141,7 +141,7 @@ noinst_PROGRAMS += \
 t_generated_code2_cxx_generate_packed_data_SOURCES = \
 	t/generated-code2/cxx-generate-packed-data.cc \
 	t/test-full.pb.cc
-t/generated-code2/cxx-generate-packed-data.$(OBJEXT): t/test-full.pb.h
+$(t_generated_code2_cxx_generate_packed_data_OBJECTS): t/test-full.pb.h
 t_generated_code2_cxx_generate_packed_data_CXXFLAGS = \
 	$(AM_CXXFLAGS) \
 	$(protobuf_CFLAGS)