$subject says it pretty much all.

The effect of _GNU_SOURCE should be pretty much equal to
_NETBSD_SOURCE, e.g. expose all extensions. There is some software
which defines _GNU_SOURCE along with _POSIX_C_SOURCE or _XOPEN_SOURCE,
which fails to build due to not exposed prototypes.

Yeah, _GNU_SOURCE is totally nonstandard, but supporting it would
help porting 3rd party software written for Linux.

Jaromir
-- 
Jaromir Dolecek <jdolecek@NetBSD.org>            http://www.NetBSD.cz/
-=- We should be mindful of the potential goal, but as the Buddhist -=-
-=- masters say, ``You may notice during meditation that you        -=-
-=- sometimes levitate or glow.   Do not let this distract you.''   -=-