[makefile] Suppress "No such file or directory" warnings at start of build

Use "-include" rather than "include" for the generated Makefile
fragments, in order to suppress the long list of warnings that
otherwise appears at the start of a clean build.

Contributed by Edward Waugh <ewaugh@netxen.com>

src/Makefile.housekeeping

 # Calculate and include the list of Makefile rules files
 #
 AUTO_DEPS	= $(patsubst %,$(BIN)/deps/%.d,$(AUTO_SRCS))
-include $(AUTO_DEPS)
+-include $(AUTO_DEPS)
 autodeps :
 	@$(ECHO) $(AUTO_DEPS)
 VERYCLEANUP	+= $(BIN)/deps
 MEDIA_DEPS		= $(patsubst %,$(BIN)/deps/%.media.d,$(AUTO_MEDIA))
 mediadeps :
 	@$(ECHO) $(MEDIA_DEPS)
-include $(MEDIA_DEPS)
+-include $(MEDIA_DEPS)
 
 # The "allXXXs" targets for each suffix
 #