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.