ITS#9215 Fix previous commit

__GLIBC__, __GNU_LIBRARY__, and so on are defined in the same header (features.h) that processes the feature-test macros. Of course that couldn't possibly work, so all !50 (merged) actually did was bring back the warning.

I'm not sure we actually can do a conditional define here, so I'm back to the _GNU_SOURCE idea, but suggestions are welcome.