Add boost's unordered_set to match the behavior of including hash_set when this boost feature is missing
Add boost's unordered_set to match the behavior of including hash_set when this boost feature is missing

(reviewed off-list)

git-svn-id: c3edda122e248ef33995da003806c930f62b9468@1424 31904243-a364-42ac-a3db-9305396518ba

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