[Intel-gfx] [PATCH i-g-t] build: Define _GNU_SOURCE in Makefile.am

[Chris Wilson]

Quoting Daniel Vetter (2017-09-05 17:19:45)
> In meson I want to just set this everywhere (no reason not to), and
> doing so will allow us to clean up a few things.

If we are keeping config.h in meson, why move -D_GNU_SOURCE to the
commandline, surely the fix is that those files not including config.h
should? Today it's a missing _GNU_SOURCE, tomorrow it could be any of the
little config details.
-Chris