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

[Jakub Jelinek]

On Thu, Mar 10, 2016 at 03:44:26AM -0500, Ed Smith-Rowland wrote:
> 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?

ENOPATCH ;)

	Jakub