Add _DEFAULT_SOURCE feature test macro.

[Rich Felker]

On Mon, Dec 23, 2013 at 02:52:56AM -0500, Mike Frysinger wrote:
> On Wednesday 18 December 2013 16:04:41 Rich Felker wrote:
> > If everybody is happy with the "_DEFAULT_SOURCE" name I'll adopt it in
> > musl too and propose that uclibc do the same too
> 
> when it comes to headers like this, uClibc just adopts what glibc does, albeit 
> at a slow pace (whenever someone gets around to it)

Yes. My point was to be an advocate for getting this adopted sooner
rather than later, but of course I can't guarantee that would actually
happen...

Rich