Add _DEFAULT_SOURCE feature test macro.

[Joseph S. Myers]

On Wed, 18 Dec 2013, Rich Felker wrote:

> To me this sounds like just about the ideal behavior. BTW I think it
> also brings glibc's defaults closer to the defaults we're providing in
> musl. We don't have a SVID switch at all, but the defaults include

I think there's some confusion here - this patch isn't meant to change the 
defaults at all.  The defaults are (BSD + SVID + POSIX.1-2008, but with 
with __USE_POSIX_IMPLICITLY defined) both before and after this patch.  
The point of this patch is to provide a way to get the default interfaces 
enabled with other options such as -std=c99 are used, (a) that doesn't 
depend on _BSD_SOURCE and _SVID_SOURCE being supported feature test macros 
and (b) that remains the same even when the default interface changes in 
future (e.g. for a newer version of POSIX).

-- 
Joseph S. Myers
joseph@codesourcery.com