move _GNU_SOURCE to the main makefile

move _GNU_SOURCE to the main makefile

A bunch of utils are relying on _GNU_SOURCE already.  The new prompt code
uses getline() which is now part of POSIX, but in older versions of glibc,
it was behind _GNU_SOURCE as it was a GNU extension.

This change doesn't actually tie us to glibc.  Only code that uses GNU
extensions does that.  It just kills warning when using older versions of
glibc.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>