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}

BUILD.gn

@@ -28,9 +28,13 @@ if (!build_with_chromium) {
     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",

webrtc.gni

@@ -77,6 +77,12 @@ declare_args() {
   # Set this to disable building with support for SCTP data channels.
   rtc_enable_sctp = true
 
+  # Set this to false to skip building examples.
+  rtc_build_examples = true
+
+  # Set this to false to skip building tools.
+  rtc_build_tools = true
+
   # Disable these to not build components which can be externally provided.
   rtc_build_json = true
   rtc_build_libsrtp = true