This removes the static dependency on X11 on Linux, allowing for
headless rendering on systems without it.

Bug 20104157

Change-Id: I463c8b3340c3ad16cd0f0d1d0f8804ee31012dd0
Reviewed-on: https://swiftshader-review.googlesource.com/2832Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>