Update floating-point feature test macro handling for C2X

[Adhemerval Zanella]

On 01/06/2021 14:30, Joseph Myers wrote:
> On Fri, 28 May 2021, Adhemerval Zanella via Libc-alpha wrote:
> 
>>> @@ -215,6 +215,13 @@ enabled.  Only some of the features from this TS are supported by
>>>  @theglibc{}.
>>>  @end defvr
>>>  
>>> +@defvr Macro __STDC_WANT_IEC_60559_EXT__
>>> +@standards{ISO, (none)}
>>> +If you define this macro, ISO C2X features defined in Annex F of that
>>> +standard are enabled.  This affects declarations of the
>>> +@code{totalorder} functions and functions related to NaN payloads.
>>> +@end defvr
>>> +
>>>  @defvr Macro _GNU_SOURCE
>>>  @standards{GNU, (none)}
>>>  If you define this macro, everything is included: @w{ISO C89}, @w{ISO
>>
>> Should it add a note for CR_DECIMAL_DIG as well? Or the gcc documentation
>> is suffice?
> 
> I think this part of the manual is only expected to cover the effects of 
> feature test macros on glibc's own headers, not on compiler-provided 
> headers.
> 

Fair enough.