Make ustring implementation safe for libc++.
Make ustring implementation safe for libc++.

Don't do the trick where we mess with the std::string internals if it's
using libc++'s headers. Eventually, we'll go back and fix this to do
something even more efficient in that case.

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