Simplify LIBDIVIDE_CONSTEXPR check

[pps83]

No description provided.

[pps83]

the comment for msvc could be simplified imo: msvc used to define __cplusplus to some ancient values even when compiling with c++14 / c++17 because they didn't have fully implemented these standards. For that reason, they had _MSVC_LANG that identifies that "incomplete" c++ version that was being targeted. Afaik, for newer version that's not needed and they properly define __cplusplus version and feature test macros. I would just put this way:

// Older msvc versions might need to check _MSVC_LANG instead of `__cpp_constexpr`

here's the info from them: https://developercommunity.visualstudio.com/t/-cplusplus-macro-is-wrong/169519#T-N223875
https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/