Build warning on Clang with -Wdeprecated-redundant-constexpr-static-def

[stephenberry]

out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated

Enabling -Wdeprecated-redundant-constexpr-static-def with warnings as errors causes fast_float to not build on Clang.

Fixing this should make the code cleaner as well.

[lemire]

@stephenberry pull request invited!

[stephenberry]

I think to support C++11 you would need macros around the old expressions. Do you want this?

[lemire]

The library is C++11 and we do not want to change this.

Using macros is acceptable.