[PATCH] manual: Update _DEFAULT_SOURCE.

[Joseph Myers]

On Mon, 19 Feb 2018, Rical Jasan wrote:

> Also, _DEFAULT_SOURCE is supposed to be defined if none of the C/POSIX
> feature test macros are defined, but the condition was lacking a test
> for _ISOC11_SOURCE, so that is also addressed.

I think this is a header bug that was user-visible in releases, so should 
be filed in Bugzilla before being fixed.

-- 
Joseph S. Myers
joseph@codesourcery.com