Move _GNU_SOURCE to gmxpre.h

This makes life simpler in files that require it, and removes clutter
from there.  The only (minor) disadvantage is that it is now defined
everywhere, so people who develop on a system where it has effect may
accidentally introduce additional function calls that depend on the
define.  But Jenkins should catch those easily.

Change-Id: Ia4ed8b5d189138694076705ded695d818bbc73e3