platform.h: define OIIO_INLINE_CONSTEXPR to be `inline constexpr` (#2832)
platform.h: define OIIO_INLINE_CONSTEXPR to be `inline constexpr` (#2832)

But only for C++ >= 17 when inline constexpr variables became available.
It defines to simply constexpr for older C++.
OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio