stop warnings if a dependency file is missing

Makefile.in

@@ -49,7 +49,7 @@ test-programs: $(TEST_PROGRAMS)
 
 .SUFFIXES: .cc .o .d
 
-%.d: %.cc
+.cc.d:
 	$(CXX) -MM -MT $(subst .cc,.o,$<) $(CXXFLAGS) $< > $@.$$$$; \
 	sed 's,\([^ :]*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \
 	rm -f $@.$$$$
@@ -96,9 +96,9 @@ install: $(PROGRAMS)
 	$(INSTALL_PROGRAM) -D thin_dump $(BINDIR)/thin_dump
 	$(INSTALL_PROGRAM) -D thin_restore $(BINDIR)/thin_restore
 
-include $(subst .cc,.d,$(SOURCE))
-include $(subst .cc,.d,$(TEST_SOURCE))
-include $(subst .cc,.d,$(PROGRAM_SOURCE))
+-include $(subst .cc,.d,$(SOURCE))
+-include $(subst .cc,.d,$(TEST_SOURCE))
+-include $(subst .cc,.d,$(PROGRAM_SOURCE))
 
 ifeq ("$(TESTING)", "yes")
 include unit-tests/Makefile.in