Slight revision of detection of C++ version (#2456)
Slight revision of detection of C++ version (#2456)

The main change is recognizing which MSVS version is C++14 compliant.
Also fixed some comment typos, and initial support for C++20 detection.
lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio