Make it build on llvm 3.5

Tried to take care to keep it compatible with <3.4.
I think this is incorrect. isfinite is only defined in namespace OIIO only for Windows. If you are using a fairly modern OIIO, you should just be able to say 'isfinite' and get the right thing -- either std on Linux/OSX, or the one in OIIO on Windows.

Note also that 'OIIO' is correct here -- it's an alias for whatever namespace is really used. It defaults to OpenImageIO, but can be overridden at build time for a particular site.