Remove the use_x11==1 condition from two things which have nothing to do
with x11: defining _GNU_SOURCE and choosing deqp_libtester_sources_unix.
This lets us build for Chrome OS where typically use_x11==0.

BUG=angleproject:1297

Change-Id: I9166f2a8119493aaab82af15e109d378fffb811a
Reviewed-on: https://chromium-review.googlesource.com/337695Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>