Make real concept constexpr

[mborland]

With C++14 we can have multi-line constexpr member functions so make real_concept support constexpr construction and operations. Should fix the build errors in #937.

[jzmaddock]

Sorry wrong fix: we want real_concept to model our absolute minimal requirements, and there are number types (for example multiprecision types which reference C library backends such as MPFR) which can never be constepxr.

[mborland]

On an unrelated note if we are going to expand constexpr support in C++14 it might be worth dropping MSVC-14.0. It was unhappy in the PRs with tables, is doing the wrong thing here, and from this pr is missing support to full C++14. For now I think we are OK, but worth being aware of.

@NAThompson for awareness

[NAThompson]

@mborland , @jzmaddock : Does MSVC do autoupdates now? Certainly on Mac it is a task of some difficulty to stay on an old AppleClang version.

[mborland]

You can still download old versions on the website https://visualstudio.microsoft.com/vs/older-downloads/. To get an older Apple Clang build I have to login with an Apple Developer account, and it still tries to update every time you launch. This was a problem when Ventura was released but the toolchains hadn't been updated yet...

[ckormanyos]

if we are going to expand constexpr support in C++14 it might be worth dropping MSVC-14.0

MSVC-14.0 cannot handle C++14 constexpr reliably. If you would like to use the acutal word constexpr without workarounds and within a C++14-compatible environment, the MSVC-14.0 must be dropped.