Make building of examples and rtc_tools optional.

If disabled, this allows running "gn gen <folder>" on a system without
GTK and other development libraries installed.

BUG=webrtc:5716

Change-Id: I1aee3d74402485113612276f6de374fb6034251f
Reviewed-on: https://webrtc-review.googlesource.com/30460
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Joachim Bauch <jbauch@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21192}
diff --git a/BUILD.gn b/BUILD.gn
index e4d3777..cfa24a1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -28,9 +28,13 @@
     testonly = true
     deps = [
       ":webrtc",
-      "examples",
-      "rtc_tools",
     ]
+    if (rtc_build_examples) {
+      deps += [ "examples" ]
+    }
+    if (rtc_build_tools) {
+      deps += [ "rtc_tools" ]
+    }
     if (rtc_include_tests) {
       deps += [
         ":rtc_unittests",