cleanup: Belatedly change OIIO_CONSTEXPR14 to constexpr

[lgritz]

We've been using C++14 minimum for years, and in fact will move to 17 for next year's major release.

Also use error reporting correctly if a sufficient C++ verion is not found

[jessey-git]

Just to double-check, the remaining uses of OIIO_CONSTEXPR14 need to remain until we bump to 3.0 since downstream apps may be using these #defines?

https://github.com/AcademySoftwareFoundation/OpenImageIO/blob/master/src/include/OpenImageIO/platform.h#L95 https://github.com/AcademySoftwareFoundation/OpenImageIO/blob/master/src/include/OpenImageIO/detail/farmhash.h#L89

[lgritz]

Just to double-check, the remaining uses of OIIO_CONSTEXPR14 need to remain until we bump to 3.0 since downstream apps may be using these #defines?

Yeah, we can remove it in 3.0 but before then we should have the define just in case somebody else downstream is using it. It's probably unlikely that anybody uses it, but we try to make the strongest possible promise that we won't make changes that will cause downstream apps to need to alter their source code.