Deqp gyp use_x11 fixes.
Deqp gyp use_x11 fixes.

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/337695
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Commit-Queue: Frank Henigman <4f3ccfa167f952ff7263448ff4377acff6a23e5c@chromium.org>

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