Add C++ special math functions to C++17

[Ed Smith-Rowland]

Here is a small patch to allow the special maths functions that came in 
with TR29124 into C++17 in Jacksonville.
I am leaving the TR29124 access with __STDCPP_WANT_MATH_SPEC_FUNCS__ for 
C++ < 17.

I added the SD-6 feature macro:
#define __cpp_lib_math_special_functions 20163L

Passes testing on x86-64-linux.

OK?