Fix TruffleRuby compile failure (#2732)
Fix TruffleRuby compile failure (#2732)

fix WERRORFLAG is nil, which is causing CI failures with current TruffleRuby master.

puma/puma,eileencodes/puma,eileencodes/puma,eileencodes/puma,puma/puma,eileencodes/puma,puma/puma,puma/puma