Update INSTALL to add libpng requirement
Update INSTALL to add libpng requirement

I'm trying to get up and running with OSL and OIIO on a fresh 
Ubuntu Lucid VM, and was unable to compile without libpng.


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