ChromeOS runs tests on some bots which run on Linux, but do not support X11.
In order to allow SwiftShader to successfully build on these bots, all X11
dependencies are #ifdefed behind the USE_X11 flag, which is already supplied
by the Chromium build system.

Change-Id: I6b914b1e662d9fbf101eb7caea7ac59e43cc7b56
Reviewed-on: https://swiftshader-review.googlesource.com/19488Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>