Removed unnecessary flags for fake.rb

Flags for ruby core such as warning and `_FORTIFY_SOURCE` macro
are not necessary to make fake.rb, except for `RUBY_EXPORT` macro
which prevents to include ruby/backward.h.

common.mk

@@ -727,7 +727,7 @@ no-fake -fake: PHONY
 # version.o depends on.
 $(arch)-fake.rb: $(srcdir)/template/fake.rb.in $(srcdir)/tool/generic_erb.rb version.$(OBJEXT) miniruby$(EXEEXT)
 	$(ECHO) generating $@
-	$(Q) $(CPP) $(warnflags) $(XCFLAGS) $(CPPFLAGS) "$(srcdir)/version.c" | \
+	$(Q) $(CPP) -DRUBY_EXPORT $(CPPFLAGS) "$(srcdir)/version.c" | \
 	$(BOOTSTRAPRUBY) "$(srcdir)/tool/generic_erb.rb" -o $@ "$(srcdir)/template/fake.rb.in" \
 		i=- srcdir="$(srcdir)" BASERUBY="$(BASERUBY)"