Fix the issue with libtiff 4 noted by arekz in issue #525.
Fix the issue with libtiff 4 noted by arekz in issue #525.

lgritz/oiio,mcanthony/oiio,OpenImageIO/oiio,micler/oiio,micler/oiio,cwilling/oiio,micler/oiio,OpenImageIO/oiio,bdeluca/oiio,scott-wilson/oiio,lgritz/oiio,cwilling/oiio,scott-wilson/oiio,bdeluca/oiio,mcanthony/oiio,bdeluca/oiio,mcanthony/oiio,lgritz/oiio,OpenImageIO/oiio,micler/oiio,OpenImageIO/oiio,lgritz/oiio,sambler/oiio,YangYangTL/oiio,YangYangTL/oiio,scott-wilson/oiio,cwilling/oiio,bdeluca/oiio,mcanthony/oiio,YangYangTL/oiio,scott-wilson/oiio,sambler/oiio,cwilling/oiio,sambler/oiio,sambler/oiio,YangYangTL/oiio