BZ #16632: Change [_BSD/_SVID]_SOURCE warning.

BZ #16632: Change [_BSD/_SVID]_SOURCE warning.

Source packages that need to support both 2.19 and
2.20 will need to decide to use _BSD_SOURCE and
_SVID_SOURCE vs. _DEFAULT_SOURCE.

The difficulty in making that decision is that
__GLIBC_MINOR__ is itself defined in features.h,
but you want to set the feature test macros before
including features.h.

Therefore to ease the transition we should disable
the warning if _DEFAULT_SOURCE is also defined.

https://sourceware.org/ml/libc-alpha/2014-02/msg00666.html

https://sourceware.org/glibc/wiki/Release/2.20#Packaging_Changes