Linux builds can assume to use X11, except if it's Android.

Change-Id: I1dc11b5fea73f198ea63d39ca924e95c1d150336
Reviewed-on: https://swiftshader-review.googlesource.com/16408Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>