Add _DEFAULT_SOURCE feature test macro.

[Mike Frysinger]

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)
-mike
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