Make building with X11 libraries optional.

Desktop capturing on Linux will be disabled in this case, but everything
can be built without any X11 development libraries installed.

BUG=webrtc:5716,webrtc:8319

Change-Id: I01bd6a4b02816b407be19476e22ff073d264b496
Reviewed-on: https://webrtc-review.googlesource.com/32360
Reviewed-by: Henrik Andreassson (OOO until Jan 2) <henrika@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Joachim Bauch <jbauch@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21462}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 7349f17..cc475f7 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -717,7 +717,7 @@
     sources += [ "macifaddrs_converter.cc" ]
   }
 
-  if (use_x11) {
+  if (rtc_use_x11) {
     libs += [
       "dl",
       "rt",