Allow overriding CFLAGS for ripper extension.

Index: ext/ripper/depend
--- ext/ripper/depend.orig
+++ ext/ripper/depend
@@ -48,6 +48,8 @@ ripper.E: ripper.c
 	$(ECHO) preprocessing ripper.c
 	$(Q) $(CC) -E $(INCFLAGS) $(CPPFLAGS) $< | $(RUBY) $(srcdir)/tools/strip.rb > $@
 
+CFLAGS += 
+
 # AUTOGENERATED DEPENDENCIES START
 ripper.o: $(RUBY_EXTCONF_H)
 ripper.o: $(arch_hdrdir)/ruby/config.h