[PING] [PATCH v2] manual: Document missing feature test macros.

[Joseph Myers]

OK, but suggested followups:

* In the description of _DEFAULT_SOURCE, note that features from the C 
standard version used by the compiler are also enabled by default.  (With 
GCC 5 and later, which defaults to -std=gnu11, that means C11 features are 
enabled, but unlike C99 they aren't part of the 2008 edition of POSIX, so 
aren't already included in the description of default features.)

* Remove the statement about C99 not being widely adopted from the 
description of _ISOC99_SOURCE.  By now, C99 is widely adopted (in the Unix 
world; I'm not sure about the state of Microsoft's compiler in that 
regard, they've historically been better at keeping up with C++ than with 
C).

* Consider obsoleting _ATFILE_SOURCE and making it into an alias for 
_POSIX_C_SOURCE = 200809L if _POSIX_C_SOURCE is undefined or has a lower 
value (similar to the _REENTRANT obsoletion); I don't think we really need 
a separate feature test macro for this subset of POSIX features.

-- 
Joseph S. Myers
joseph@codesourcery.com