Not all Linux platforms will have X11 available (Wayland, GGP), so we have to
declare a variable that checks use_x11 separately.

Bug: angleproject:4116
Change-Id: I7e33956da6344b8db4d4e32dbbd1b42f58f7019c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1919632Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>